From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 80E35C433F5 for ; Mon, 25 Oct 2021 14:56:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5D40A60C40 for ; Mon, 25 Oct 2021 14:56:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233822AbhJYO6f (ORCPT ); Mon, 25 Oct 2021 10:58:35 -0400 Received: from mail.kernel.org ([198.145.29.99]:46902 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233720AbhJYO57 (ORCPT ); Mon, 25 Oct 2021 10:57:59 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 6FDD860C40 for ; Mon, 25 Oct 2021 14:55:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1635173736; bh=bQPttLQ+qazBVGcAEMA4emicUB27s6vpb6pOtHC929Y=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=i8QZDVK3qpF7DzSnX7Cb9ax0pkJ9ZcJVVtWif74puF2r/s19uPuUvz/vVW/ET+3WQ aYrroH9BFtLbgxJj+/LYsVUD5Imkfa6AQeRLwhixZReGyRzYzJDTKluZUf/wcYzEBz Ksl9rTNS9pfuC23ARQH5VDKpBq7nuiuNm+hQyd2kqBYmekCEGqGxx9xKd1zz0agD/X mIJcztEMQuA9+ENE3e5D0uaH6cSQPpYd+xMDhoX1alfIMT3d92jyMoPos+Us2304/v 03AAKrBTxw/LcrGrZdH+/9V/WTls42F2GnDuAhXT8GYmUp7hwMCwbIibCfMxV2Abml k8EnITg8qf6iw== Received: by mail-qk1-f177.google.com with SMTP id bp7so11861320qkb.12 for ; Mon, 25 Oct 2021 07:55:36 -0700 (PDT) X-Gm-Message-State: AOAM5315dKrDDbPok6wnIMHfsoMR645Xk28TdhTxssvMgj1Ya/YRFh8I CFUZn6XDG5aJtoVVOGfPyak12+8xV1gomR+zjXE= X-Google-Smtp-Source: ABdhPJwHmEALf7wYrBPJ0RPZsJulMu7I51IFwEdus1fB0ElszeoBZ+ODYGRBOQUBYaffbRbU5n7j18LYKG/HmkH83h0= X-Received: by 2002:a37:943:: with SMTP id 64mr14093149qkj.163.1635173735485; Mon, 25 Oct 2021 07:55:35 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Filipe Manana Date: Mon, 25 Oct 2021 15:54:59 +0100 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2] btrfs: fix deadlock due to page faults during direct IO reads and writes To: Josef Bacik Cc: linux-btrfs , Wang Yugui , Filipe Manana Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Mon, Oct 25, 2021 at 3:42 PM Josef Bacik wrote: > > 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(). Yes, I had noticed that. But since the patchset was not yet merged, I decided to leave that for later. > > > + /* > > + * 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. Nop, the iomap code does the +=, see: https://lore.kernel.org/linux-fsdevel/20211019134204.3382645-15-agruenba@redhat.com/ > > > > > @@ -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, Nop, same as above. Thanks. > > Josef