kernel/git/stable/linux-stable.git/mm/page-writeback.c, branch linux-4.14.y

mm/page-writeback.c: avoid potential division by zero in wb_min_max_ratio()

2020-01-23T07:20:32+00:00

commit 6d9e8c651dd979aa666bee15f086745f3ea9c4b3 upstream. Patch series "use div64_ul() instead of div_u64() if the divisor is unsigned long". We were first inspired by commit b0ab99e7736a ("sched: Fix possible divide by zero in avg_atom () calculation"), then refer to the recently analyzed mm code, we found this suspicious place. 201 if (min) { 202 min *= this_bw; 203 do_div(min, tot_bw); 204 } And we also disassembled and confirmed it: /usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 201 0xffffffff811c37da <__wb_calc_thresh+234>: xor %r10d,%r10d 0xffffffff811c37dd <__wb_calc_thresh+237>: test %rax,%rax 0xffffffff811c37e0 <__wb_calc_thresh+240>: je 0xffffffff811c3800 <__wb_calc_thresh+272> /usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 202 0xffffffff811c37e2 <__wb_calc_thresh+242>: imul %r8,%rax /usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 203 0xffffffff811c37e6 <__wb_calc_thresh+246>: mov %r9d,%r10d ---> truncates it to 32 bits here 0xffffffff811c37e9 <__wb_calc_thresh+249>: xor %edx,%edx 0xffffffff811c37eb <__wb_calc_thresh+251>: div %r10 0xffffffff811c37ee <__wb_calc_thresh+254>: imul %rbx,%rax 0xffffffff811c37f2 <__wb_calc_thresh+258>: shr $0x2,%rax 0xffffffff811c37f6 <__wb_calc_thresh+262>: mul %rcx 0xffffffff811c37f9 <__wb_calc_thresh+265>: shr $0x2,%rdx 0xffffffff811c37fd <__wb_calc_thresh+269>: mov %rdx,%r10 This series uses div64_ul() instead of div_u64() if the divisor is unsigned long, to avoid truncation to 32-bit on 64-bit platforms. This patch (of 3): The variables 'min' and 'max' are unsigned long and do_div truncates them to 32 bits, which means it can test non-zero and be truncated to zero for division. Fix this issue by using div64_ul() instead. Link: http://lkml.kernel.org/r/20200102081442.8273-2-wenyang@linux.alibaba.com Fixes: 693108a8a667 ("writeback: make bdi->min/max_ratio handling cgroup writeback aware") Signed-off-by: Wen Yang Reviewed-by: Andrew Morton Cc: Qian Cai Cc: Tejun Heo Cc: Jens Axboe Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman

mm/page-writeback.c: fix range_cyclic writeback vs writepages deadlock

2019-12-01T08:13:48+00:00

[ Upstream commit 64081362e8ff4587b4554087f3cfc73d3e0a4cd7 ] We've recently seen a workload on XFS filesystems with a repeatable deadlock between background writeback and a multi-process application doing concurrent writes and fsyncs to a small range of a file. range_cyclic writeback Process 1 Process 2 xfs_vm_writepages write_cache_pages writeback_index = 2 cycled = 0 .... find page 2 dirty lock Page 2 ->writepage page 2 writeback page 2 clean page 2 added to bio no more pages write() locks page 1 dirties page 1 locks page 2 dirties page 1 fsync() .... xfs_vm_writepages write_cache_pages start index 0 find page 1 towrite lock Page 1 ->writepage page 1 writeback page 1 clean page 1 added to bio find page 2 towrite lock Page 2 page 2 is writeback write() locks page 1 dirties page 1 fsync() .... xfs_vm_writepages write_cache_pages start index 0 !done && !cycled sets index to 0, restarts lookup find page 1 dirty find page 1 towrite lock Page 1 page 1 is writeback lock Page 1 DEADLOCK because: - process 1 needs page 2 writeback to complete to make enough progress to issue IO pending for page 1 - writeback needs page 1 writeback to complete so process 2 can progress and unlock the page it is blocked on, then it can issue the IO pending for page 2 - process 2 can't make progress until process 1 issues IO for page 1 The underlying cause of the problem here is that range_cyclic writeback is processing pages in descending index order as we hold higher index pages in a structure controlled from above write_cache_pages(). The write_cache_pages() caller needs to be able to submit these pages for IO before write_cache_pages restarts writeback at mapping index 0 to avoid wcp inverting the page lock/writeback wait order. generic_writepages() is not susceptible to this bug as it has no private context held across write_cache_pages() - filesystems using this infrastructure always submit pages in ->writepage immediately and so there is no problem with range_cyclic going back to mapping index 0. However: mpage_writepages() has a private bio context, exofs_writepages() has page_collect fuse_writepages() has fuse_fill_wb_data nfs_writepages() has nfs_pageio_descriptor xfs_vm_writepages() has xfs_writepage_ctx All of these ->writepages implementations can hold pages under writeback in their private structures until write_cache_pages() returns, and hence they are all susceptible to this deadlock. Also worth noting is that ext4 has it's own bastardised version of write_cache_pages() and so it /may/ have an equivalent deadlock. I looked at the code long enough to understand that it has a similar retry loop for range_cyclic writeback reaching the end of the file and then promptly ran away before my eyes bled too much. I'll leave it for the ext4 developers to determine if their code is actually has this deadlock and how to fix it if it has. There's a few ways I can see avoid this deadlock. There's probably more, but these are the first I've though of: 1. get rid of range_cyclic altogether 2. range_cyclic always stops at EOF, and we start again from writeback index 0 on the next call into write_cache_pages() 2a. wcp also returns EAGAIN to ->writepages implementations to indicate range cyclic has hit EOF. writepages implementations can then flush the current context and call wpc again to continue. i.e. lift the retry into the ->writepages implementation 3. range_cyclic uses trylock_page() rather than lock_page(), and it skips pages it can't lock without blocking. It will already do this for pages under writeback, so this seems like a no-brainer 3a. all non-WB_SYNC_ALL writeback uses trylock_page() to avoid blocking as per pages under writeback. I don't think #1 is an option - range_cyclic prevents frequently dirtied lower file offset from starving background writeback of rarely touched higher file offsets. #2 is simple, and I don't think it will have any impact on performance as going back to the start of the file implies an immediate seek. We'll have exactly the same number of seeks if we switch writeback to another inode, and then come back to this one later and restart from index 0. #2a is pretty much "status quo without the deadlock". Moving the retry loop up into the wcp caller means we can issue IO on the pending pages before calling wcp again, and so avoid locking or waiting on pages in the wrong order. I'm not convinced we need to do this given that we get the same thing from #2 on the next writeback call from the writeback infrastructure. #3 is really just a band-aid - it doesn't fix the access/wait inversion problem, just prevents it from becoming a deadlock situation. I'd prefer we fix the inversion, not sweep it under the carpet like this. #3a is really an optimisation that just so happens to include the band-aid fix of #3. So it seems that the simplest way to fix this issue is to implement solution #2 Link: http://lkml.kernel.org/r/20181005054526.21507-1-david@fromorbit.com Signed-off-by: Dave Chinner Reviewed-by: Jan Kara Cc: Nicholas Piggin Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Sasha Levin

mm/page-writeback.c: don't break integrity writeback on ->writepage() error

2019-01-26T08:37:06+00:00

[ Upstream commit 3fa750dcf29e8606e3969d13d8e188cc1c0f511d ] write_cache_pages() is used in both background and integrity writeback scenarios by various filesystems. Background writeback is mostly concerned with cleaning a certain number of dirty pages based on various mm heuristics. It may not write the full set of dirty pages or wait for I/O to complete. Integrity writeback is responsible for persisting a set of dirty pages before the writeback job completes. For example, an fsync() call must perform integrity writeback to ensure data is on disk before the call returns. write_cache_pages() unconditionally breaks out of its processing loop in the event of a ->writepage() error. This is fine for background writeback, which had no strict requirements and will eventually come around again. This can cause problems for integrity writeback on filesystems that might need to clean up state associated with failed page writeouts. For example, XFS performs internal delayed allocation accounting before returning a ->writepage() error, where applicable. If the current writeback happens to be associated with an unmount and write_cache_pages() completes the writeback prematurely due to error, the filesystem is unmounted in an inconsistent state if dirty+delalloc pages still exist. To handle this problem, update write_cache_pages() to always process the full set of pages for integrity writeback regardless of ->writepage() errors. Save the first encountered error and return it to the caller once complete. This facilitates XFS (or any other fs that expects integrity writeback to process the entire set of dirty pages) to clean up its internal state completely in the event of persistent mapping errors. Background writeback continues to exit on the first error encountered. [akpm@linux-foundation.org: fix typo in comment] Link: http://lkml.kernel.org/r/20181116134304.32440-1-bfoster@redhat.com Signed-off-by: Brian Foster Reviewed-by: Jan Kara Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Sasha Levin

writeback: safer lock nesting

2018-04-24T07:36:39+00:00

commit 2e898e4c0a3897ccd434adac5abb8330194f527b upstream. lock_page_memcg()/unlock_page_memcg() use spin_lock_irqsave/restore() if the page's memcg is undergoing move accounting, which occurs when a process leaves its memcg for a new one that has memory.move_charge_at_immigrate set. unlocked_inode_to_wb_begin,end() use spin_lock_irq/spin_unlock_irq() if the given inode is switching writeback domains. Switches occur when enough writes are issued from a new domain. This existing pattern is thus suspicious: lock_page_memcg(page); unlocked_inode_to_wb_begin(inode, &locked); ... unlocked_inode_to_wb_end(inode, locked); unlock_page_memcg(page); If both inode switch and process memcg migration are both in-flight then unlocked_inode_to_wb_end() will unconditionally enable interrupts while still holding the lock_page_memcg() irq spinlock. This suggests the possibility of deadlock if an interrupt occurs before unlock_page_memcg(). truncate __cancel_dirty_page lock_page_memcg unlocked_inode_to_wb_begin unlocked_inode_to_wb_end end_page_writeback test_clear_page_writeback lock_page_memcg unlock_page_memcg Due to configuration limitations this deadlock is not currently possible because we don't mix cgroup writeback (a cgroupv2 feature) and memory.move_charge_at_immigrate (a cgroupv1 feature). If the kernel is hacked to always claim inode switching and memcg moving_account, then this script triggers lockup in less than a minute: cd /mnt/cgroup/memory mkdir a b echo 1 > a/memory.move_charge_at_immigrate echo 1 > b/memory.move_charge_at_immigrate ( echo $BASHPID > a/cgroup.procs while true; do dd if=/dev/zero of=/mnt/big bs=1M count=256 done ) & while true; do sync done & sleep 1h & SLEEP=$! while true; do echo $SLEEP > a/cgroup.procs echo $SLEEP > b/cgroup.procs done The deadlock does not seem possible, so it's debatable if there's any reason to modify the kernel. I suggest we should to prevent future surprises. And Wang Long said "this deadlock occurs three times in our environment", so there's more reason to apply this, even to stable. Stable 4.4 has minor conflicts applying this patch. For a clean 4.4 patch see "[PATCH for-4.4] writeback: safer lock nesting" https://lkml.org/lkml/2018/4/11/146 Wang Long said "this deadlock occurs three times in our environment" [gthelen@google.com: v4] Link: http://lkml.kernel.org/r/20180411084653.254724-1-gthelen@google.com [akpm@linux-foundation.org: comment tweaks, struct initialization simplification] Change-Id: Ibb773e8045852978f6207074491d262f1b3fb613 Link: http://lkml.kernel.org/r/20180410005908.167976-1-gthelen@google.com Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates") Signed-off-by: Greg Thelen Reported-by: Wang Long Acked-by: Wang Long Acked-by: Michal Hocko Reviewed-by: Andrew Morton Cc: Johannes Weiner Cc: Tejun Heo Cc: Nicholas Piggin Cc: [v4.2+] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds [natechancellor: Adjust context due to lack of b93b016313b3b] Signed-off-by: Nathan Chancellor Signed-off-by: Greg Kroah-Hartman

mm: rename global_page_state to global_zone_page_state

2017-09-07T00:27:29+00:00

global_page_state is error prone as a recent bug report pointed out [1]. It only returns proper values for zone based counters as the enum it gets suggests. We already have global_node_page_state so let's rename global_page_state to global_zone_page_state to be more explicit here. All existing users seems to be correct: $ git grep "global_page_state(NR_" | sed 's@.*($NR_[A-Z_]*$).*@\1@' | sort | uniq -c 2 NR_BOUNCE 2 NR_FREE_CMA_PAGES 11 NR_FREE_PAGES 1 NR_KERNEL_STACK_KB 1 NR_MLOCK 2 NR_PAGETABLE This patch shouldn't introduce any functional change. [1] http://lkml.kernel.org/r/201707260628.v6Q6SmaS030814@www262.sakura.ne.jp Link: http://lkml.kernel.org/r/20170801134256.5400-2-hannes@cmpxchg.org Signed-off-by: Michal Hocko Signed-off-by: Johannes Weiner Cc: Tetsuo Handa Cc: Josef Bacik Cc: Vladimir Davydov Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds

mm: memcontrol: fix NULL pointer crash in test_clear_page_writeback()

2017-08-18T22:32:01+00:00

Jaegeuk and Brad report a NULL pointer crash when writeback ending tries to update the memcg stats: BUG: unable to handle kernel NULL pointer dereference at 00000000000003b0 IP: test_clear_page_writeback+0x12e/0x2c0 [...] RIP: 0010:test_clear_page_writeback+0x12e/0x2c0 Call Trace: end_page_writeback+0x47/0x70 f2fs_write_end_io+0x76/0x180 [f2fs] bio_endio+0x9f/0x120 blk_update_request+0xa8/0x2f0 scsi_end_request+0x39/0x1d0 scsi_io_completion+0x211/0x690 scsi_finish_command+0xd9/0x120 scsi_softirq_done+0x127/0x150 __blk_mq_complete_request_remote+0x13/0x20 flush_smp_call_function_queue+0x56/0x110 generic_smp_call_function_single_interrupt+0x13/0x30 smp_call_function_single_interrupt+0x27/0x40 call_function_single_interrupt+0x89/0x90 RIP: 0010:native_safe_halt+0x6/0x10 (gdb) l *(test_clear_page_writeback+0x12e) 0xffffffff811bae3e is in test_clear_page_writeback (./include/linux/memcontrol.h:619). 614 mod_node_page_state(page_pgdat(page), idx, val); 615 if (mem_cgroup_disabled() || !page->mem_cgroup) 616 return; 617 mod_memcg_state(page->mem_cgroup, idx, val); 618 pn = page->mem_cgroup->nodeinfo[page_to_nid(page)]; 619 this_cpu_add(pn->lruvec_stat->count[idx], val); 620 } 621 622 unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order, 623 gfp_t gfp_mask, The issue is that writeback doesn't hold a page reference and the page might get freed after PG_writeback is cleared (and the mapping is unlocked) in test_clear_page_writeback(). The stat functions looking up the page's node or zone are safe, as those attributes are static across allocation and free cycles. But page->mem_cgroup is not, and it will get cleared if we race with truncation or migration. It appears this race window has been around for a while, but less likely to trigger when the memcg stats were updated first thing after PG_writeback is cleared. Recent changes reshuffled this code to update the global node stats before the memcg ones, though, stretching the race window out to an extent where people can reproduce the problem. Update test_clear_page_writeback() to look up and pin page->mem_cgroup before clearing PG_writeback, then not use that pointer afterward. It is a partial revert of 62cccb8c8e7a ("mm: simplify lock_page_memcg()") but leaves the pageref-holding callsites that aren't affected alone. Link: http://lkml.kernel.org/r/20170809183825.GA26387@cmpxchg.org Fixes: 62cccb8c8e7a ("mm: simplify lock_page_memcg()") Signed-off-by: Johannes Weiner Reported-by: Jaegeuk Kim Tested-by: Jaegeuk Kim Reported-by: Bradley Bolen Tested-by: Brad Bolen Cc: Vladimir Davydov Cc: Michal Hocko Cc: [4.6+] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds

writeback: rework wb_[dec|inc]_stat family of functions

2017-07-12T23:26:05+00:00

Currently the writeback statistics code uses a percpu counters to hold various statistics. Furthermore we have 2 families of functions - those which disable local irq and those which doesn't and whose names begin with double underscore. However, they both end up calling __add_wb_stats which in turn calls percpu_counter_add_batch which is already irq-safe. Exploiting this fact allows to eliminated the __wb_* functions since they don't add any further protection than we already have. Furthermore, refactor the wb_* function to call __add_wb_stat directly without the irq-disabling dance. This will likely result in better runtime of code which deals with modifying the stat counters. While at it also document why percpu_counter_add_batch is in fact preempt and irq-safe since at least 3 people got confused. Link: http://lkml.kernel.org/r/1498029937-27293-1-git-send-email-nborisov@suse.com Signed-off-by: Nikolay Borisov Acked-by: Tejun Heo Reviewed-by: Jan Kara Cc: Josef Bacik Cc: Mel Gorman Cc: Jeff Layton Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds

Merge tag 'for-linus-v4.13-1' of git://git.kernel.org/pub/scm/linux/kernel/git/jlayton/linux

2017-07-08T01:39:15+00:00

Pull Writeback error handling fixes from Jeff Layton: "The main rationale for all of these changes is to tighten up writeback error reporting to userland. There are many ways now that writeback errors can be lost, such that fsync/fdatasync/msync return 0 when writeback actually failed. This pile contains a small set of cleanups and writeback error handling fixes that I was able to break off from the main pile (#2). Two of the patches in this pile are trivial. The exceptions are the patch to fix up error handling in write_one_page, and the patch to make JFS pay attention to write_one_page errors" * tag 'for-linus-v4.13-1' of git://git.kernel.org/pub/scm/linux/kernel/git/jlayton/linux: fs: remove call_fsync helper function mm: clean up error handling in write_one_page JFS: do not ignore return code from write_one_page() mm: drop "wait" parameter from write_one_page()

mm: memcontrol: per-lruvec stats infrastructure

2017-07-06T23:24:35+00:00

lruvecs are at the intersection of the NUMA node and memcg, which is the scope for most paging activity. Introduce a convenient accounting infrastructure that maintains statistics per node, per memcg, and the lruvec itself. Then convert over accounting sites for statistics that are already tracked in both nodes and memcgs and can be easily switched. [hannes@cmpxchg.org: fix crash in the new cgroup stat keeping code] Link: http://lkml.kernel.org/r/20170531171450.GA10481@cmpxchg.org [hannes@cmpxchg.org: don't track uncharged pages at all Link: http://lkml.kernel.org/r/20170605175254.GA8547@cmpxchg.org [hannes@cmpxchg.org: add missing free_percpu()] Link: http://lkml.kernel.org/r/20170605175354.GB8547@cmpxchg.org [linux@roeck-us.net: hexagon: fix build error caused by include file order] Link: http://lkml.kernel.org/r/20170617153721.GA4382@roeck-us.net Link: http://lkml.kernel.org/r/20170530181724.27197-6-hannes@cmpxchg.org Signed-off-by: Johannes Weiner Signed-off-by: Guenter Roeck Acked-by: Vladimir Davydov Cc: Josef Bacik Cc: Michal Hocko Cc: Rik van Riel Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds

mm: clean up error handling in write_one_page

2017-07-05T22:44:22+00:00

Don't try to check PageError since that's potentially racy and not necessarily going to be set after writepage errors out. Instead, check the mapping for an error after writepage returns. That should also help us detect errors that occurred if the VM tried to clean the page earlier due to memory pressure. Signed-off-by: Jeff Layton Reviewed-by: Jan Kara