Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache
From: Song Liu <song@kernel.org>
Date: 2021-09-29 23:42:06
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On Wed, Sep 29, 2021 at 10:56 AM Matthew Wilcox [off-list ref] wrote:
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Now, I am able to crash the system on find_lock_entries () { ... VM_BUG_ON_PAGE(page->index != xas.xa_index, page); } I guess it is related. I will test more.That's a bogus VM_BUG_ON. I have a patch in my tree to delete it. Andrew has it too, but for some reason, he hasn't sent it on to Linus.+++ b/mm/filemap.c@@ -2093,7 +2093,6 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start, if (!xa_is_value(page)) { if (page->index < start) goto put; - VM_BUG_ON_PAGE(page->index != xas.xa_index, page); if (page->index + thp_nr_pages(page) - 1 > end) goto put; if (!trylock_page(page))
Yes, after removing this line, I am able to see the same bug. Here is my finding so far: The issue is NOT caused by concurrent khugepaged:collapse_file() and truncate_pagecache(inode, 0). With some printks, we can see a clear time gap (>2 second ) between collapse_file() finishes, and truncate_pagecache() (which crashes soon). Therefore, my earlier suggestion that adds deny_write_access() to collapse_file() does NOT work. The crash is actually caused by concurrent truncate_pagecache(inode, 0). If I change the number of write thread in stress_madvise_dso.c to one, (IOW, one thread_read and one thread_write), I cannot reproduce the crash anymore. I think this means we cannot fix this issue in collapse_file(), because it finishes long before the crash. Thanks, Song