From patchwork Mon Feb 10 18:12:24 2025 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: "Raphael S. Carvalho" X-Patchwork-Id: 13968336 Received: from mail-pj1-f46.google.com (mail-pj1-f46.google.com [209.85.216.46]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E7A5D255E4B for ; Mon, 10 Feb 2025 18:12:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.46 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739211163; cv=none; b=fFh18sqZvrr0R+Ha6k0g0LRD48/u424IQzAAuil9HG4heLMe0kYC/0R/sF6GzOsK9GV6ZW3sa9uZ64XiBSo28IcmHYqQjkAC3+G0VvcXQSruFIQJDQPc81LtwowLwdl5qFi7u4Tp5093MYjYsAfcKjgeoynn32KRBnonB87dyEM= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739211163; c=relaxed/simple; bh=3/MHfhlR7A1fMCKuzyQ9kYjcRJa0otc+shpPn/Aicas=; h=MIME-Version:From:Date:Message-ID:Subject:To:Cc:Content-Type; b=tVQ8ieMQKFMGrZiHlnmvdXvZ33Fs/0IT1hfFcx5/25JmycsaOsc9Dn9KD0AGK7czIFlobgnslqTY66mqron73hxXcUjP3gzU8wzzjM/DA/7JS9rg/BfVKdmr2EQJ/ZwIqGQiQh3ouXlQeVW5p6A0sGOnGIF9lrBaNiL/npFtzqA= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=scylladb.com; spf=pass smtp.mailfrom=scylladb.com; dkim=pass (2048-bit key) header.d=scylladb.com header.i=@scylladb.com header.b=z0b8llt8; arc=none smtp.client-ip=209.85.216.46 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=scylladb.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=scylladb.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=scylladb.com header.i=@scylladb.com header.b="z0b8llt8" Received: by mail-pj1-f46.google.com with SMTP id 98e67ed59e1d1-2f441791e40so6469758a91.3 for ; Mon, 10 Feb 2025 10:12:41 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=scylladb.com; s=google; t=1739211161; x=1739815961; darn=vger.kernel.org; h=content-transfer-encoding:cc:to:subject:message-id:date:from :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=ghqe50qYZAx8/Up2NeOnbz1uVUWHYq1gR6XXZUhiI3o=; b=z0b8llt83pPx5PjRHlWIT7qmb5sy1rAJz8lWlbXxjsxf2997pHjrpVh9l1c8qCvvLf 3H3fGGd1Ct4HGcCmGjba0UAOTxveLF0yFRGbIny8AERJ3ScAOJ7kbwpa3TtZ2HnRfFYi +OG6mrocqNnj3NLvLSu/Xk8Je8wPNOB9utiNESqwULsaqiyKZHU+Fv2fR6nkTpincS0w kooryDLcPvjhIDePU8YnhtLluGxBj4YOsUlZCgWNIK/d9e7dOwb8BECvakXyIrlHs4jy c0OZrbTVdy4XAfoAU4rAckhgQIWDKaCMSCLyEPxg7Z1GfkkIz4RWhhaOO15IcHvLWrvM +3nQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1739211161; x=1739815961; h=content-transfer-encoding:cc:to:subject:message-id:date:from :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=ghqe50qYZAx8/Up2NeOnbz1uVUWHYq1gR6XXZUhiI3o=; b=FBby0D2gmgyq1K9Hwwl7u48WUEw/sKlQebF0vSgEHs2cwlQqsLo8QLjU8WIEP8CSzh SSHyRG24iID3IWFxRsYZRBjSp5NjxhIC6jMlQDEAsr1hqG9wExPAeBYHzsvuchBkegBv ZpW+2Epv4c8uJnQ9pUCHGFskvdq2/9YYLkKZHaGcZRABLvSF7iCbP0aBG9Ml0Htobkym BhPGyJfkm3CAJtohPlJKU4lYigFZrWm7FXH0IRN+MK1Zi+tjRO3cL/hxPs0JbLBXh589 fceuyUZWiGkeEk7OGWwHF1VO13WIzz5EY+HSYOOP3yiQgpxPnSeu0ocMRjzo6JirS4s2 iN9A== X-Gm-Message-State: AOJu0Yxj6+SioqL9mguf7YvA3Rk2sGXbPPp8U/Hajf5JfnoBN9FL7+j1 v9wthzIOeBV54Pmf0YDWp9nmnl56FGT3cDX2pjQmjOhj0Vqwd8uZlNmBT79EpgT87DPSUDzpTWN QO1aoADIeQdMiGwMXgJVAxGDvPVWB+FVAn2o6biAkZ+YyUWmx3oZGt0uggEwdh62tu96vFD9n0/ MYKx6UumXLClczD5fBqr0n1nHh1YaqbjHqk1YJZSRjXJ8CLGk/f4HBrDqwFAmsRmmUFj3CSMlVF V7+bSO1ynFfx/l0J9RVYx0sQZsSmH6aTlNYGv9x6hMrEK8U/8EdcpxKIHf+14Knv0IO6vOyU1Z2 0oKMb2WLjiPnh2hfF9GIAzhrbE8oc/nKuUkXihUri3rVSJIQc7jH2A6VA4LJWyriyvjhjhneSHj ZDL+QRPrRF0LEfXWY75QezeIw X-Gm-Gg: ASbGncvbeL4Lfmk7YFSI8ptJ2ntMOad3W53GNCO/owbkJ4Zf+LvaMjsBilaN7kR9x5N zN8KDEQJqSsgrR6d3RQGDcMl1ne3B7ufqHuEYXvy+ckarVE/7biR0W2GObXbnL3lraRq+dYzNME MFeHty88qlSUp3ig== X-Google-Smtp-Source: AGHT+IH42tWjnTVpToH+j5t5Jo4/pmVQaB9GJtJldKzz+C6W598q1wQBxNOoLHTh4L4xQClrCInTELzbqnmG1Vghncw= X-Received: by 2002:a17:90b:2e47:b0:2fa:20f4:d277 with SMTP id 98e67ed59e1d1-2fa243e39dbmr22125692a91.24.1739211160478; Mon, 10 Feb 2025 10:12:40 -0800 (PST) Precedence: bulk X-Mailing-List: linux-xfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: "Raphael S. Carvalho" Date: Mon, 10 Feb 2025 15:12:24 -0300 X-Gm-Features: AWEUYZla75N_T1omj1dgRRrPxpzZF8m_so82a-s0ma_JD4_WjgtvDUIb2E1F1E8 Message-ID: Subject: Possible regression with buffered writes + NOWAIT behavior, under memory pressure To: linux-xfs@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org Cc: djwong@kernel.org, Dave Chinner , hch@lst.de, Avi Kivity X-CLOUD-SEC-AV-Sent: true X-CLOUD-SEC-AV-Info: scylladb,google_mail,monitor X-Gm-Spam: 0 X-Gm-Phishy: 0 X-CLOUD-SEC-AV-Sent: true X-CLOUD-SEC-AV-Info: scylla,google_mail,monitor X-Gm-Spam: 0 X-Gm-Phishy: 0 While running scylladb test suite, which uses io_uring + buffered writes + XFS, the system was spuriously returning ENOMEM, despite there being plenty of available memory to be reclaimed from the page cache. FWIW, I am running: 6.12.9-100.fc40.x86_64 Tracing showed io_uring_complete failing the request with ENOMEM: # cat /sys/kernel/debug/tracing/trace | grep "result -12" -B 100 | grep "0000000065b91cd1" reactor-1-707139 [000] ..... 46737.358518: io_uring_submit_req: ring 00000000e52339b8, req 0000000065b91cd1, user_data 0x50f0001e4000, opcode WRITE, flags 0x200000, sq_thread 0 reactor-1-707139 [000] ..... 46737.358526: io_uring_file_get: ring 00000000e52339b8, req 0000000065b91cd1, user_data 0x50f0001e4000, fd 45 reactor-1-707139 [000] ...1. 46737.358560: io_uring_complete: ring 00000000e52339b8, req 0000000065b91cd1, user_data 0x50f0001e4000, result -12, cflags 0x0 extra1 0 extra2 0 That puzzled me. Using retsnoop, it pointed to iomap_get_folio: 00:34:16.180612 -> 00:34:16.180651 TID/PID 253786/253721 (reactor-1/combined_tests): entry_SYSCALL_64_after_hwframe+0x76 do_syscall_64+0x82 __do_sys_io_uring_enter+0x265 io_submit_sqes+0x209 io_issue_sqe+0x5b io_write+0xdd xfs_file_buffered_write+0x84 iomap_file_buffered_write+0x1a6 32us [-ENOMEM] iomap_write_begin+0x408 iter=&{.inode=0xffff8c67aa031138,.len=4096,.flags=33,.iomap={.addr=0xffffffffffffffff,.length=4096,.type=1,.flags=3,.bdev=0x… pos=0 len=4096 foliop=0xffffb32c296b7b80 ! 4us [-ENOMEM] iomap_get_folio iter=&{.inode=0xffff8c67aa031138,.len=4096,.flags=33,.iomap={.addr=0xffffffffffffffff,.length=4096,.type=1,.flags=3,.bdev=0x… pos=0 len=4096 Another trace shows iomap_file_buffered_write with ki_flags 2359304, which translate into (IOCB_WRITE & IOCB_ALLOC_CACHE & IOCB_NOWAIT) And flags 33 in iomap_get_folio means IOMAP_NOWAIT, which makes sense since XFS translates IOCB_NOWAIT into IOMAP_NOWAIT for performing the buffered write through iomap subsystem: fs/iomap/buffered-io.c- if (iocb->ki_flags & IOCB_NOWAIT) fs/iomap/buffered-io.c: iter.flags |= IOMAP_NOWAIT; We know io_uring works by first attempting to write with IOCB_NOWAIT, and if it fails with EAGAIN, it falls back to worker thread without the NOWAIT semantics. iomap_get_folio(), once called with IOMAP_NOWAIT, will request the allocation to follow GFP_NOWAIT behavior, so allocation can potentially fail under pressure. Coming across 'iomap: Add async buffered write support', I see Darrick wrote: "FGP_NOWAIT can cause __filemap_get_folio to return a NULL folio, which makes iomap_write_begin return -ENOMEM. If nothing has been written yet, won't that cause the ENOMEM to escape to userspace? Why do we want that instead of EAGAIN?" In the patch ''mm: return an ERR_PTR from __filemap_get_folio', I see the following changes: folio = filemap_alloc_folio(alloc_gfp, order); Am I missing something? Regards, Raphael --- a/fs/iomap/buffered-io.c +++ b/fs/iomap/buffered-io.c @@ -468,19 +468,12 @@ EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate); struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos) { unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS; - struct folio *folio; if (iter->flags & IOMAP_NOWAIT) fgp |= FGP_NOWAIT; - folio = __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT, + return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT, fgp, mapping_gfp_mask(iter->inode->i_mapping)); - if (folio) - return folio; - - if (iter->flags & IOMAP_NOWAIT) - return ERR_PTR(-EAGAIN); - return ERR_PTR(-ENOMEM); } This leads to me believe we have a regression in this area, after that patch, since iomap_get_folio() is no longer returning EAGAIN with IOMAP_NOWAIT, if __filemap_get_folio() failed to get a folio. Now it returns ENOMEM unconditionally. Since we pushed the error picking decision to __filemap_get_folio, I think it makes sense for us to patch it such that it returns EAGAIN if allocation failed (under pressure) because IOMAP_NOWAIT was requested by its caller and allocation is not allowed to block waiting for reclaimer to do its thing. A possible way to fix it is this one-liner, but I am not well versed in this area, so someone may end up suggesting a better fix: diff --git a/mm/filemap.c b/mm/filemap.c index 804d7365680c..9e698a619545 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1964,7 +1964,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index, do { gfp_t alloc_gfp = gfp; - err = -ENOMEM; + err = (fgp_flags & FGP_NOWAIT) ? -ENOMEM : -EAGAIN; if (order > min_order) alloc_gfp |= __GFP_NORETRY | __GFP_NOWARN;