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 X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2547DC432C1 for ; Tue, 24 Sep 2019 19:42:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D789B21783 for ; Tue, 24 Sep 2019 19:42:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=cmpxchg-org.20150623.gappssmtp.com header.i=@cmpxchg-org.20150623.gappssmtp.com header.b="Kaa8D01n" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2441929AbfIXTmm (ORCPT ); Tue, 24 Sep 2019 15:42:42 -0400 Received: from mail-qk1-f194.google.com ([209.85.222.194]:42725 "EHLO mail-qk1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2441488AbfIXTml (ORCPT ); Tue, 24 Sep 2019 15:42:41 -0400 Received: by mail-qk1-f194.google.com with SMTP id f16so3051357qkl.9 for ; Tue, 24 Sep 2019 12:42:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cmpxchg-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=TMciHmBxHBQDxyP+Eh+I154TeCeJbBEgcjucUWxN/D0=; b=Kaa8D01nxH31BY/wU1r4Y14uNM+Tkf9xBu0lA6+ubSaYpvT6frXQYrBgbuMJQQqehS 7TIiW3k/q72rVwFPiY6FtgItUs92dyablCd+wIa92fSWPErBIM3MIH9cf7DUQDUEQaLC +cwpbjBG4uBDui0S82iQVbz3XguCwKQ644Rv+LUNzk6Zot2Z4fpBrsQu7RJwJSZ2LS0L nu+ULRswP69lsXHtxHL4P4mAAM3PQvmBsVC2lZKNH9s3Z/X9VP7BAmyRCg1eDD7/65oD mH/UPwXJAN3TX90hq3LLvn7GJleI6kUXuG2T8qNATOVTtXmmxevLXKXkU0GLRxGu4+kE iIPQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=TMciHmBxHBQDxyP+Eh+I154TeCeJbBEgcjucUWxN/D0=; b=Wxv3q4E/+G8zQQ94a0/yFcEIfUtT8pqOQwfC5hdKeIASicWAZOY+fF7t14ydLY+0qb 9ofi+oyxsS0IUiarDrtpaYlSJP+H81Am87Iv/D9KCo2NGb6H23TUDsxQWYqNbkkfnV+e 4KWK5A+hIAFo/SWuQRr2lHrhlkEDwAlX0XJyWZWIWAjhv5xvK+ocRPev1OSjG4/faPgR CZ0EmD1B36xXcLm0zCEK8i3p7fQcuBbdpBKVWFqv8Yl2xjONhRn1Hb9bcIX8SCWnilwc Lc4nlI06XmiIb26SxA+R/eIPnS6l4lyTLsL5y6Or6UyZe//WqReNHTrZ2d/t4Scqrvo8 5s3w== X-Gm-Message-State: APjAAAXI4EtG3NofZkvYlFIAozySubJ40u4Ydpko1A3UIWgZLVd0CFPE vnG1AoKhEeQ5JYbR3y18UyamOEgC/Bc= X-Google-Smtp-Source: APXvYqwmixPwLaGzBg6yXlhJamU8ogD8OKXEaJ/g1PkbV9sgFWUKBzennCFpy+4sp5ElfzNfn9QM5w== X-Received: by 2002:a37:6292:: with SMTP id w140mr4250295qkb.24.1569354160079; Tue, 24 Sep 2019 12:42:40 -0700 (PDT) Received: from localhost ([2620:10d:c091:500::a51]) by smtp.gmail.com with ESMTPSA id z141sm1516042qka.126.2019.09.24.12.42.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 24 Sep 2019 12:42:39 -0700 (PDT) Date: Tue, 24 Sep 2019 15:42:38 -0400 From: Johannes Weiner To: Matthew Wilcox Cc: Andrew Morton , Josef Bacik , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: drop mmap_sem before calling balance_dirty_pages() in write fault Message-ID: <20190924194238.GA29030@cmpxchg.org> References: <20190924171518.26682-1-hannes@cmpxchg.org> <20190924174809.GH1855@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190924174809.GH1855@bombadil.infradead.org> User-Agent: Mutt/1.12.1 (2019-06-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 24, 2019 at 10:48:09AM -0700, Matthew Wilcox wrote: > On Tue, Sep 24, 2019 at 01:15:18PM -0400, Johannes Weiner wrote: > > +static int fault_dirty_shared_page(struct vm_fault *vmf) > > vm_fault_t, shirley? Ah yes, I changed it. > > @@ -2239,16 +2241,36 @@ static void fault_dirty_shared_page(struct vm_area_struct *vma, > > mapping = page_rmapping(page); > > unlock_page(page); > > > > + if (!page_mkwrite) > > + file_update_time(vma->vm_file); > > + > > + /* > > + * Throttle page dirtying rate down to writeback speed. > > + * > > + * mapping may be NULL here because some device drivers do not > > + * set page.mapping but still dirty their pages > > + * > > + * Drop the mmap_sem before waiting on IO, if we can. The file > > + * is pinning the mapping, as per above. > > + */ > > if ((dirtied || page_mkwrite) && mapping) { > > - /* > > - * Some device drivers do not set page.mapping > > - * but still dirty their pages > > - */ > > + struct file *fpin = NULL; > > + > > + if ((vmf->flags & > > + (FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT)) == > > + FAULT_FLAG_ALLOW_RETRY) { > > + fpin = get_file(vma->vm_file); > > + up_read(&vma->vm_mm->mmap_sem); > > + ret = VM_FAULT_RETRY; > > + } > > + > > balance_dirty_pages_ratelimited(mapping); > > + > > + if (fpin) > > + fput(fpin); > > } > > > > - if (!page_mkwrite) > > - file_update_time(vma->vm_file); > > + return ret; > > } > > I'm not a fan of moving file_update_time() to _before_ the > balance_dirty_pages call. Can you elaborate why? If the filesystem has a page_mkwrite op, it will have already called file_update_time() before this function is entered. If anything, this change makes the sequence more consistent. > Also, this is now the third place that needs > maybe_unlock_mmap_for_io, see > https://lore.kernel.org/linux-mm/20190917120852.x6x3aypwvh573kfa@box/ Good idea, I moved the helper to internal.h and converted to it. I left the shmem site alone, though. It doesn't require the file pinning, so it shouldn't pointlessly bump the file refcount and suggest such a dependency - that could cost somebody later quite a bit of time trying to understand the code. > How about: > > + struct file *fpin = NULL; > > if ((dirtied || page_mkwrite) && mapping) { > fpin = maybe_unlock_mmap_for_io(vmf, fpin); > balance_dirty_pages_ratelimited(mapping); > } > + > + if (fpin) { > + file_update_time(fpin); This isn't an equivalent change and could update the time twice if the fs has a page_mkwrite op. > + fput(fpin); > + return VM_FAULT_RETRY; > + } > > if (!page_mkwrite) > file_update_time(vma->vm_file); > + return 0; > } > > > /* > > @@ -2491,6 +2513,7 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf) > > __releases(vmf->ptl) > > { > > struct vm_area_struct *vma = vmf->vma; > > + int ret = VM_FAULT_WRITE; > > vm_fault_t again. Done. > > @@ -3576,7 +3599,6 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf) > > static vm_fault_t do_fault(struct vm_fault *vmf) > > { > > struct vm_area_struct *vma = vmf->vma; > > - struct mm_struct *vm_mm = vma->vm_mm; > > vm_fault_t ret; > > > > /* > > @@ -3617,7 +3639,12 @@ static vm_fault_t do_fault(struct vm_fault *vmf) > > > > /* preallocated pagetable is unused: free it */ > > if (vmf->prealloc_pte) { > > - pte_free(vm_mm, vmf->prealloc_pte); > > + /* > > + * XXX: Accessing vma->vm_mm now is not safe. The page > > + * fault handler may have dropped the mmap_sem a long > > + * time ago. Only s390 derefs that parameter. > > + */ > > + pte_free(vma->vm_mm, vmf->prealloc_pte); > > I'm confused. This code looks like it was safe before (as it was caching > vma->vm_mm in a local variable), and now you've made it unsafe ... ? You're right. After looking at it again, this was a total brainfart: the mm itself is of course safe to access. I removed these two hunks. Thanks! Updated patch: --- >From 60c1493bd3b5e0800147165a1dc36b77cf3306e5 Mon Sep 17 00:00:00 2001 From: Johannes Weiner Date: Wed, 8 May 2019 13:53:38 -0700 Subject: [PATCH v2] mm: drop mmap_sem before calling balance_dirty_pages() in write fault One of our services is observing hanging ps/top/etc under heavy write IO, and the task states show this is an mmap_sem priority inversion: A write fault is holding the mmap_sem in read-mode and waiting for (heavily cgroup-limited) IO in balance_dirty_pages(): [<0>] balance_dirty_pages+0x724/0x905 [<0>] balance_dirty_pages_ratelimited+0x254/0x390 [<0>] fault_dirty_shared_page.isra.96+0x4a/0x90 [<0>] do_wp_page+0x33e/0x400 [<0>] __handle_mm_fault+0x6f0/0xfa0 [<0>] handle_mm_fault+0xe4/0x200 [<0>] __do_page_fault+0x22b/0x4a0 [<0>] page_fault+0x45/0x50 [<0>] 0xffffffffffffffff Somebody tries to change the address space, contending for the mmap_sem in write-mode: [<0>] call_rwsem_down_write_failed_killable+0x13/0x20 [<0>] do_mprotect_pkey+0xa8/0x330 [<0>] SyS_mprotect+0xf/0x20 [<0>] do_syscall_64+0x5b/0x100 [<0>] entry_SYSCALL_64_after_hwframe+0x3d/0xa2 [<0>] 0xffffffffffffffff The waiting writer locks out all subsequent readers to avoid lock starvation, and several threads can be seen hanging like this: [<0>] call_rwsem_down_read_failed+0x14/0x30 [<0>] proc_pid_cmdline_read+0xa0/0x480 [<0>] __vfs_read+0x23/0x140 [<0>] vfs_read+0x87/0x130 [<0>] SyS_read+0x42/0x90 [<0>] do_syscall_64+0x5b/0x100 [<0>] entry_SYSCALL_64_after_hwframe+0x3d/0xa2 [<0>] 0xffffffffffffffff To fix this, do what we do for cache read faults already: drop the mmap_sem before calling into anything IO bound, in this case the balance_dirty_pages() function, and return VM_FAULT_RETRY. Signed-off-by: Johannes Weiner --- mm/filemap.c | 21 --------------------- mm/internal.h | 21 +++++++++++++++++++++ mm/memory.c | 38 +++++++++++++++++++++++++++----------- 3 files changed, 48 insertions(+), 32 deletions(-) version 2: - use maybe_unlock_mmap_for_io(), as per Willy - remove vma dereference bug, as per Willy - use vm_fault_t, as per Willy diff --git a/mm/filemap.c b/mm/filemap.c index 7f99d53005bb..9072c3fbf837 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2362,27 +2362,6 @@ EXPORT_SYMBOL(generic_file_read_iter); #ifdef CONFIG_MMU #define MMAP_LOTSAMISS (100) -static struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf, - struct file *fpin) -{ - int flags = vmf->flags; - - if (fpin) - return fpin; - - /* - * FAULT_FLAG_RETRY_NOWAIT means we don't want to wait on page locks or - * anything, so we only pin the file and drop the mmap_sem if only - * FAULT_FLAG_ALLOW_RETRY is set. - */ - if ((flags & (FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT)) == - FAULT_FLAG_ALLOW_RETRY) { - fpin = get_file(vmf->vma->vm_file); - up_read(&vmf->vma->vm_mm->mmap_sem); - } - return fpin; -} - /* * lock_page_maybe_drop_mmap - lock the page, possibly dropping the mmap_sem * @vmf - the vm_fault for this fault. diff --git a/mm/internal.h b/mm/internal.h index 0d5f720c75ab..7dd7fbb577a9 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -362,6 +362,27 @@ vma_address(struct page *page, struct vm_area_struct *vma) return max(start, vma->vm_start); } +static inline struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf, + struct file *fpin) +{ + int flags = vmf->flags; + + if (fpin) + return fpin; + + /* + * FAULT_FLAG_RETRY_NOWAIT means we don't want to wait on page locks or + * anything, so we only pin the file and drop the mmap_sem if only + * FAULT_FLAG_ALLOW_RETRY is set. + */ + if ((flags & (FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT)) == + FAULT_FLAG_ALLOW_RETRY) { + fpin = get_file(vmf->vma->vm_file); + up_read(&vmf->vma->vm_mm->mmap_sem); + } + return fpin; +} + #else /* !CONFIG_MMU */ static inline void clear_page_mlock(struct page *page) { } static inline void mlock_vma_page(struct page *page) { } diff --git a/mm/memory.c b/mm/memory.c index 2e796372927f..088b6d3b6f88 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2221,10 +2221,11 @@ static vm_fault_t do_page_mkwrite(struct vm_fault *vmf) * * The function expects the page to be locked and unlocks it. */ -static void fault_dirty_shared_page(struct vm_area_struct *vma, - struct page *page) +static vm_fault_t fault_dirty_shared_page(struct vm_fault *vmf) { + struct vm_area_struct *vma = vmf->vma; struct address_space *mapping; + struct page *page = vmf->page; bool dirtied; bool page_mkwrite = vma->vm_ops && vma->vm_ops->page_mkwrite; @@ -2239,16 +2240,30 @@ static void fault_dirty_shared_page(struct vm_area_struct *vma, mapping = page_rmapping(page); unlock_page(page); + if (!page_mkwrite) + file_update_time(vma->vm_file); + + /* + * Throttle page dirtying rate down to writeback speed. + * + * mapping may be NULL here because some device drivers do not + * set page.mapping but still dirty their pages + * + * Drop the mmap_sem before waiting on IO, if we can. The file + * is pinning the mapping, as per above. + */ if ((dirtied || page_mkwrite) && mapping) { - /* - * Some device drivers do not set page.mapping - * but still dirty their pages - */ + struct file *fpin; + + fpin = maybe_unlock_mmap_for_io(vmf, NULL); balance_dirty_pages_ratelimited(mapping); + if (fpin) { + fput(fpin); + return VM_FAULT_RETRY; + } } - if (!page_mkwrite) - file_update_time(vma->vm_file); + return 0; } /* @@ -2491,6 +2506,7 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf) __releases(vmf->ptl) { struct vm_area_struct *vma = vmf->vma; + vm_fault_t ret = VM_FAULT_WRITE; get_page(vmf->page); @@ -2514,10 +2530,10 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf) wp_page_reuse(vmf); lock_page(vmf->page); } - fault_dirty_shared_page(vma, vmf->page); + ret |= fault_dirty_shared_page(vmf); put_page(vmf->page); - return VM_FAULT_WRITE; + return ret; } /* @@ -3561,7 +3577,7 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf) return ret; } - fault_dirty_shared_page(vma, vmf->page); + ret |= fault_dirty_shared_page(vmf); return ret; } -- 2.23.0