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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id d14sm3289999otq.65.2021.10.04.19.58.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 04 Oct 2021 19:58:11 -0700 (PDT) Date: Mon, 4 Oct 2021 19:58:10 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: Rongwei Wang cc: Song Liu , Matthew Wilcox , Hugh Dickins , Andrew Morton , Linux MM , Linux Kernel Mailing List , William Kucharski Subject: Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache In-Reply-To: <8d8fb192-bd8d-8a08-498d-ca7204d4a716@linux.alibaba.com> Message-ID: References: <20210923194343.ca0f29e1c4d361170343a6f2@linux-foundation.org> <9e41661d-9919-d556-8c49-610dae157553@linux.alibaba.com> <68737431-01d2-e6e3-5131-7d7c731e49ae@linux.alibaba.com> <8d8fb192-bd8d-8a08-498d-ca7204d4a716@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 5 Oct 2021, Rongwei Wang wrote: > Hi, > I have run our cases these two days to stress test new Patch #1. The new Patch > #1 mainly add filemap_invalidate_{un}lock before and after > truncate_pagecache(), basing on original Patch #1. And the crash has not > happened. > > Now, I keep the original Patch #1, then adding the code below which suggested > by liu song (I'm not sure which one I should add in the next version, > Suggested-by or Signed-off-by? If you know, please remind me). > > - if (filemap_nr_thps(inode->i_mapping)) > + if (filemap_nr_thps(inode->i_mapping)) { > + filemap_invalidate_lock(inode->i_mapping); > truncate_pagecache(inode, 0); > + filemap_invalidate_unlock(inode->i_mapping); > + } I won't NAK that patch; but I still believe it's unnecessary, and don't see how it protects against all the races (collapse_file() does not use that lock, whereas collapse_file() does use page lock). And if you're hoping to fix 5.10, then you will have to backport those invalidate_lock patches there too (they're really intended to protect hole-punching). > > And the reason for keeping the original Patch #1 is mainly to fix the race > between collapse_file and truncate_pagecache. It seems necessary. Despite the > two-day test, I did not reproduce this race any more. > > In addition, I also test the below method: > > diff --git a/mm/truncate.c b/mm/truncate.c > index 3f47190f98a8..33604e4ce60a 100644 > --- a/mm/truncate.c > +++ b/mm/truncate.c > @@ -210,8 +210,6 @@ invalidate_complete_page(struct address_space *mapping, > struct page *page) > > int truncate_inode_page(struct address_space *mapping, struct page *page) > { > - VM_BUG_ON_PAGE(PageTail(page), page); > - > if (page->mapping != mapping) > return -EIO; > > I am not very sure this VM_BUG_ON_PAGE(PageTail) is what Hugh means. And > the test results show that only removing this VM_BUG_ON_PAGE(PageTail) has no > effect. So, I still keep the original Patch #1 to fix one race. Yes, that's exactly what I meant, and thank you for intending to try it. But if that patch had "no effect", then I think you were not running the kernel with that patch applied: because it deletes the BUG on line 213 of mm/truncate.c, which is what you reported in the first mail! Or, is line 213 of mm/truncate.c in your 5.10.46-hugetext+ kernel something else? I've been looking at 5.15-rc. But I wasn't proposing to delete it merely to hide the BUG: as I hope I explained, we could move it below the page->mapping check, but it wouldn't really be of any value there since tails have NULL page->mapping anyway (well, I didn't check first and second tails, maybe mapping gets reused for some compound page field in those). I was proposing to delete it because the page->mapping check then weeds out the racy case once we're holding page lock, without the need for adding anything special. Hugh