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[174.109.172.136]) by smtp.gmail.com with ESMTPSA id b2sm8466129qtg.88.2021.10.25.07.42.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 25 Oct 2021 07:42:01 -0700 (PDT) Date: Mon, 25 Oct 2021 10:42:00 -0400 From: Josef Bacik To: fdmanana@kernel.org Cc: linux-btrfs@vger.kernel.org, wangyugui@e16-tech.com, Filipe Manana Subject: Re: [PATCH v2] btrfs: fix deadlock due to page faults during direct IO reads and writes Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Mon, Oct 25, 2021 at 10:42:59AM +0100, fdmanana@kernel.org wrote: > From: Filipe Manana > > If we do a direct IO read or write when the buffer given by the user is > memory mapped to the file range we are going to do IO, we end up ending > in a deadlock. This is triggered by the new test case generic/647 from > fstests. > > For a direct IO read we get a trace like this: > > [ 967.872718] INFO: task mmap-rw-fault:12176 blocked for more than 120 seconds. > [ 967.874161] Not tainted 5.14.0-rc7-btrfs-next-95 #1 > [ 967.874909] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > [ 967.875983] task:mmap-rw-fault state:D stack: 0 pid:12176 ppid: 11884 flags:0x00000000 > [ 967.875992] Call Trace: > [ 967.875999] __schedule+0x3ca/0xe10 > [ 967.876015] schedule+0x43/0xe0 > [ 967.876020] wait_extent_bit.constprop.0+0x1eb/0x260 [btrfs] > [ 967.876109] ? do_wait_intr_irq+0xb0/0xb0 > [ 967.876118] lock_extent_bits+0x37/0x90 [btrfs] > [ 967.876150] btrfs_lock_and_flush_ordered_range+0xa9/0x120 [btrfs] > [ 967.876184] ? extent_readahead+0xa7/0x530 [btrfs] > [ 967.876214] extent_readahead+0x32d/0x530 [btrfs] > [ 967.876253] ? lru_cache_add+0x104/0x220 > [ 967.876255] ? kvm_sched_clock_read+0x14/0x40 > [ 967.876258] ? sched_clock_cpu+0xd/0x110 > [ 967.876263] ? lock_release+0x155/0x4a0 > [ 967.876271] read_pages+0x86/0x270 > [ 967.876274] ? lru_cache_add+0x125/0x220 > [ 967.876281] page_cache_ra_unbounded+0x1a3/0x220 > [ 967.876291] filemap_fault+0x626/0xa20 > [ 967.876303] __do_fault+0x36/0xf0 > [ 967.876308] __handle_mm_fault+0x83f/0x15f0 > [ 967.876322] handle_mm_fault+0x9e/0x260 > [ 967.876327] __get_user_pages+0x204/0x620 > [ 967.876332] ? get_user_pages_unlocked+0x69/0x340 > [ 967.876340] get_user_pages_unlocked+0xd3/0x340 > [ 967.876349] internal_get_user_pages_fast+0xbca/0xdc0 > [ 967.876366] iov_iter_get_pages+0x8d/0x3a0 > [ 967.876374] bio_iov_iter_get_pages+0x82/0x4a0 > [ 967.876379] ? lock_release+0x155/0x4a0 > [ 967.876387] iomap_dio_bio_actor+0x232/0x410 > [ 967.876396] iomap_apply+0x12a/0x4a0 > [ 967.876398] ? iomap_dio_rw+0x30/0x30 > [ 967.876414] __iomap_dio_rw+0x29f/0x5e0 > [ 967.876415] ? iomap_dio_rw+0x30/0x30 > [ 967.876420] ? lock_acquired+0xf3/0x420 > [ 967.876429] iomap_dio_rw+0xa/0x30 > [ 967.876431] btrfs_file_read_iter+0x10b/0x140 [btrfs] > [ 967.876460] new_sync_read+0x118/0x1a0 > [ 967.876472] vfs_read+0x128/0x1b0 > [ 967.876477] __x64_sys_pread64+0x90/0xc0 > [ 967.876483] do_syscall_64+0x3b/0xc0 > [ 967.876487] entry_SYSCALL_64_after_hwframe+0x44/0xae > [ 967.876490] RIP: 0033:0x7fb6f2c038d6 > [ 967.876493] RSP: 002b:00007fffddf586b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000011 > [ 967.876496] RAX: ffffffffffffffda RBX: 0000000000001000 RCX: 00007fb6f2c038d6 > [ 967.876498] RDX: 0000000000001000 RSI: 00007fb6f2c17000 RDI: 0000000000000003 > [ 967.876499] RBP: 0000000000001000 R08: 0000000000000003 R09: 0000000000000000 > [ 967.876501] R10: 0000000000001000 R11: 0000000000000246 R12: 0000000000000003 > [ 967.876502] R13: 0000000000000000 R14: 00007fb6f2c17000 R15: 0000000000000000 > > This happens because at btrfs_dio_iomap_begin() we lock the extent range > and return with it locked - we only unlock in the endio callback, at > end_bio_extent_readpage() -> endio_readpage_release_extent(). Then after > iomap called the btrfs_dio_iomap_begin() callback, it triggers the page > faults that resulting in reading the pages, through the readahead callback > btrfs_readahead(), and through there we end to attempt to lock again the > same extent range (or a subrange of what we locked before), resulting in > the deadlock. > > For a direct IO write, the scenario is a bit different, and it results in > trace like this: > > [ 1132.442520] run fstests generic/647 at 2021-08-31 18:53:35 > [ 1330.349355] INFO: task mmap-rw-fault:184017 blocked for more than 120 seconds. > [ 1330.350540] Not tainted 5.14.0-rc7-btrfs-next-95 #1 > [ 1330.351158] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > [ 1330.351900] task:mmap-rw-fault state:D stack: 0 pid:184017 ppid:183725 flags:0x00000000 > [ 1330.351906] Call Trace: > [ 1330.351913] __schedule+0x3ca/0xe10 > [ 1330.351930] schedule+0x43/0xe0 > [ 1330.351935] btrfs_start_ordered_extent+0x108/0x1c0 [btrfs] > [ 1330.352020] ? do_wait_intr_irq+0xb0/0xb0 > [ 1330.352028] btrfs_lock_and_flush_ordered_range+0x8c/0x120 [btrfs] > [ 1330.352064] ? extent_readahead+0xa7/0x530 [btrfs] > [ 1330.352094] extent_readahead+0x32d/0x530 [btrfs] > [ 1330.352133] ? lru_cache_add+0x104/0x220 > [ 1330.352135] ? kvm_sched_clock_read+0x14/0x40 > [ 1330.352138] ? sched_clock_cpu+0xd/0x110 > [ 1330.352143] ? lock_release+0x155/0x4a0 > [ 1330.352151] read_pages+0x86/0x270 > [ 1330.352155] ? lru_cache_add+0x125/0x220 > [ 1330.352162] page_cache_ra_unbounded+0x1a3/0x220 > [ 1330.352172] filemap_fault+0x626/0xa20 > [ 1330.352176] ? filemap_map_pages+0x18b/0x660 > [ 1330.352184] __do_fault+0x36/0xf0 > [ 1330.352189] __handle_mm_fault+0x1253/0x15f0 > [ 1330.352203] handle_mm_fault+0x9e/0x260 > [ 1330.352208] __get_user_pages+0x204/0x620 > [ 1330.352212] ? get_user_pages_unlocked+0x69/0x340 > [ 1330.352220] get_user_pages_unlocked+0xd3/0x340 > [ 1330.352229] internal_get_user_pages_fast+0xbca/0xdc0 > [ 1330.352246] iov_iter_get_pages+0x8d/0x3a0 > [ 1330.352254] bio_iov_iter_get_pages+0x82/0x4a0 > [ 1330.352259] ? lock_release+0x155/0x4a0 > [ 1330.352266] iomap_dio_bio_actor+0x232/0x410 > [ 1330.352275] iomap_apply+0x12a/0x4a0 > [ 1330.352278] ? iomap_dio_rw+0x30/0x30 > [ 1330.352292] __iomap_dio_rw+0x29f/0x5e0 > [ 1330.352294] ? iomap_dio_rw+0x30/0x30 > [ 1330.352306] btrfs_file_write_iter+0x238/0x480 [btrfs] > [ 1330.352339] new_sync_write+0x11f/0x1b0 > [ 1330.352344] ? NF_HOOK_LIST.constprop.0.cold+0x31/0x3e > [ 1330.352354] vfs_write+0x292/0x3c0 > [ 1330.352359] __x64_sys_pwrite64+0x90/0xc0 > [ 1330.352365] do_syscall_64+0x3b/0xc0 > [ 1330.352369] entry_SYSCALL_64_after_hwframe+0x44/0xae > [ 1330.352372] RIP: 0033:0x7f4b0a580986 > [ 1330.352379] RSP: 002b:00007ffd34d75418 EFLAGS: 00000246 ORIG_RAX: 0000000000000012 > [ 1330.352382] RAX: ffffffffffffffda RBX: 0000000000001000 RCX: 00007f4b0a580986 > [ 1330.352383] RDX: 0000000000001000 RSI: 00007f4b0a3a4000 RDI: 0000000000000003 > [ 1330.352385] RBP: 00007f4b0a3a4000 R08: 0000000000000003 R09: 0000000000000000 > [ 1330.352386] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003 > [ 1330.352387] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000 > > Unlike for reads, at btrfs_dio_iomap_begin() we return with the extent > range unlocked, but later when the page faults are triggered and we try > to read the extents, we end up btrfs_lock_and_flush_ordered_range() where > we find the ordered extent for our write, created by the iomap callback > btrfs_dio_iomap_begin(), and we wait for it to complete, which makes us > deadlock since we can't complete the ordered extent without reading the > pages (the iomap code only submits the bio after the pages are faulted > in). > > Fix this by setting the nofault attribute of the given iov_iter and retry > the direct IO read/write if we get an -EFAULT error returned from iomap. > For reads, also disable page faults completely, this is because when we > read from a hole or a prealloc extent, we can still trigger page faults > due to the call to iov_iter_zero() done by iomap - at the momemnt, it is > oblivious to the value of the ->nofault attribute of an iov_iter. > We also need to keep track of the number of bytes written or read, and > pass it to iomap_dio_rw(), as well as use the new flag IOMAP_DIO_PARTIAL. > > This depends on the iov_iter and iomap changes done by a recent patchset > from Andreas Gruenbacher, which is not yet merged to Linus' tree at the > moment of this writing. The cover letter has the following subject: > > "[PATCH v8 00/19] gfs2: Fix mmap + page fault deadlocks" > > The thread can be found at: > > https://lore.kernel.org/linux-fsdevel/20211019134204.3382645-1-agruenba@redhat.com/ > > Fixing these issues could be done without the iov_iter and iomap changes > introduced in that patchset, however it would be much more complex due to > the need of reordering some operations for writes and having to be able > to pass some state through nested and deep call chains, which would be > particularly cumbersome for reads - for example make the readahead and > the endio handlers for page reads be aware we are in a direct IO read > context and know which inode and extent range we locked before. > > Signed-off-by: Filipe Manana > --- > > V2: Updated read path to fallback to buffered IO in case it's taking a long > time to make any progress, fixing an issue reported by Wang Yugui. > Rebased on the v8 patchset on which it depends on and on current misc-next. > > fs/btrfs/file.c | 137 ++++++++++++++++++++++++++++++++++++++++++------ > 1 file changed, 121 insertions(+), 16 deletions(-) > > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c > index 581662d16b72..58c94205d325 100644 > --- a/fs/btrfs/file.c > +++ b/fs/btrfs/file.c > @@ -1912,16 +1912,17 @@ static ssize_t check_direct_IO(struct btrfs_fs_info *fs_info, > > static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > { > + const bool is_sync_write = (iocb->ki_flags & IOCB_DSYNC); > struct file *file = iocb->ki_filp; > struct inode *inode = file_inode(file); > struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb); > loff_t pos; > ssize_t written = 0; > ssize_t written_buffered; > + size_t prev_left = 0; > loff_t endbyte; > ssize_t err; > unsigned int ilock_flags = 0; > - struct iomap_dio *dio = NULL; > > if (iocb->ki_flags & IOCB_NOWAIT) > ilock_flags |= BTRFS_ILOCK_TRY; > @@ -1964,23 +1965,79 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > goto buffered; > } > > - dio = __iomap_dio_rw(iocb, from, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > - 0, 0); Since we're the only users of this (and iomap_dio_complete) you could have some follow-up patches that get rid of these helpers and the export of iomap_dio_complete(). > + /* > + * We remove IOCB_DSYNC so that we don't deadlock when iomap_dio_rw() > + * calls generic_write_sync() (through iomap_dio_complete()), because > + * that results in calling fsync (btrfs_sync_file()) which will try to > + * lock the inode in exclusive/write mode. > + */ > + if (is_sync_write) > + iocb->ki_flags &= ~IOCB_DSYNC; > > - btrfs_inode_unlock(inode, ilock_flags); > + /* > + * The iov_iter can be mapped to the same file range we are writing to. > + * If that's the case, then we will deadlock in the iomap code, because > + * it first calls our callback btrfs_dio_iomap_begin(), which will create > + * an ordered extent, and after that it will fault in the pages that the > + * iov_iter refers to. During the fault in we end up in the readahead > + * pages code (starting at btrfs_readahead()), which will lock the range, > + * find that ordered extent and then wait for it to complete (at > + * btrfs_lock_and_flush_ordered_range()), resulting in a deadlock since > + * obviously the ordered extent can never complete as we didn't submit > + * yet the respective bio(s). This always happens when the buffer is > + * memory mapped to the same file range, since the iomap DIO code always > + * invalidates pages in the target file range (after starting and waiting > + * for any writeback). > + * > + * So here we disable page faults in the iov_iter and then retry if we > + * got -EFAULT, faulting in the pages before the retry. > + */ > +again: > + from->nofault = true; > + err = iomap_dio_rw(iocb, from, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > + IOMAP_DIO_PARTIAL, written); > + from->nofault = false; > > - if (IS_ERR_OR_NULL(dio)) { > - err = PTR_ERR_OR_ZERO(dio); > - if (err < 0 && err != -ENOTBLK) > - goto out; > - } else { > - written = iomap_dio_complete(dio); > + if (err > 0) > + written = err; Should this be written += err; if the first guy is a partial write and then the next one we get is full we'll lose the first part of the partial write. > @@ -3668,10 +3727,56 @@ static ssize_t btrfs_direct_read(struct kiocb *iocb, struct iov_iter *to) > return 0; > > btrfs_inode_lock(inode, BTRFS_ILOCK_SHARED); > +again: > + /* > + * This is similar to what we do for direct IO writes, see the comment > + * at btrfs_direct_write(), but we also disable page faults in addition > + * to disabling them only at the iov_iter level. This is because when > + * reading from a hole or prealloc extent, iomap calls iov_iter_zero(), > + * which can still trigger page fault ins despite having set ->nofault > + * to true of our 'to' iov_iter. > + * > + * The difference to direct IO writes is that we deadlock when trying > + * to lock the extent range in the inode's tree during he page reads > + * triggered by the fault in (while for writes it is due to waiting for > + * our own ordered extent). This is because for direct IO reads, > + * btrfs_dio_iomap_begin() returns with the extent range locked, which > + * is only unlocked in the endio callback (end_bio_extent_readpage()). > + */ > + pagefault_disable(); > + to->nofault = true; > ret = iomap_dio_rw(iocb, to, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > - 0, 0); > + IOMAP_DIO_PARTIAL, read); > + to->nofault = false; > + pagefault_enable(); > + > + if (ret > 0) > + read = ret; > + Same here, this should be read += ret; right? Thanks, Josef