Thanks Johannes! > On Nov 1, 2019, at 11:31 AM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Wed, Oct 30, 2019 at 06:51:15PM -0700, Andrew Morton wrote: >> To that end, here's the combination of >> mmthp-recheck-each-page-before-collapsing-file-thp.patch and this >> patch: >> >> From: Song Liu <songliubraving@xxxxxx> >> Subject: mm,thp: recheck each page before collapsing file THP >> >> In collapse_file(), for !is_shmem case, current check cannot guarantee >> the locked page is up-to-date. Specifically, xas_unlock_irq() should >> not be called before lock_page() and get_page(); and it is necessary to >> recheck PageUptodate() after locking the page. >> >> With this bug and CONFIG_READ_ONLY_THP_FOR_FS=y, madvise(HUGE)'ed .text >> may contain corrupted data. This is because khugepaged mistakenly >> collapses some not up-to-date sub pages into a huge page, and assumes >> the huge page is up-to-date. This will NOT corrupt data in the disk, >> because the page is read-only and never written back. Fix this by >> properly checking PageUptodate() after locking the page. This check >> replaces "VM_BUG_ON_PAGE(!PageUptodate(page), page);". >> >> Also, move PageDirty() check after locking the page. Current >> khugepaged should not try to collapse dirty file THP, because it is >> limited to read-only .text. Add a warning with the PageDirty() check >> as it should not happen. This warning is added after page_mapping() >> check, because if the page is truncated, it might be dirty. >> >> [songliubraving@xxxxxx: flush file for !is_shmem PageDirty() case in collapse_file()] >> Link: http://lkml.kernel.org/r/20191030200736.3455046-1-songliubraving@xxxxxx >> [songliubraving@xxxxxx: v4] >> Link: http://lkml.kernel.org/r/20191022191006.411277-1-songliubraving@xxxxxx >> [songliubraving@xxxxxx: fix deadlock in collapse_file()] >> Link: http://lkml.kernel.org/r/20191028221414.3685035-1-songliubraving@xxxxxx >> Link: http://lkml.kernel.org/r/20191018180345.4188310-1-songliubraving@xxxxxx >> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS") >> Signed-off-by: Song Liu <songliubraving@xxxxxx> >> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> >> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> >> Cc: Hugh Dickins <hughd@xxxxxxxxxx> >> Cc: William Kucharski <william.kucharski@xxxxxxxxxx> >> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> >> --- >> >> mm/khugepaged.c | 49 +++++++++++++++++++++++++++++++++++++--------- >> 1 file changed, 40 insertions(+), 9 deletions(-) >> >> --- a/mm/khugepaged.c~mmthp-recheck-each-page-before-collapsing-file-thp >> +++ a/mm/khugepaged.c >> @@ -1601,17 +1601,33 @@ static void collapse_file(struct mm_stru >> result = SCAN_FAIL; >> goto xa_unlocked; >> } >> - } else if (!PageUptodate(page)) { >> - xas_unlock_irq(&xas); >> - wait_on_page_locked(page); >> - if (!trylock_page(page)) { >> - result = SCAN_PAGE_LOCK; >> - goto xa_unlocked; >> + if (WARN_ON_ONCE(PageDirty(page))) { >> + /* >> + * page from readahead should not >> + * be dirty. Show warning if this >> + * somehow happens. >> + */ >> + result = SCAN_FAIL; >> + goto out_unlock; > > I'm not sure we need this check here. We have a dirty check on the > locked page below, and it's not clear why there is a suspicion of > readahead producing dirty pages...? I was thinking whether we can remove the if (!is_shmem && PageDirty(page)) { xxxx } check after this. But I agree we can keep it. > >> } >> - get_page(page); >> } else if (PageDirty(page)) { >> + /* >> + * khugepaged only works on read-only fd, >> + * so this page is dirty because it hasn't >> + * been flushed since first write. There >> + * won't be new dirty pages. >> + * >> + * Trigger async flush here and hope the >> + * writeback is done when khugepaged >> + * revisits this page. >> + * >> + * This is a one-off situation. We are not >> + * forcing writeback in loop. >> + */ >> + xas_unlock_irq(&xas); >> + filemap_flush(mapping); >> result = SCAN_FAIL; >> - goto xa_locked; >> + goto xa_unlocked; > > We should do this to improve the file THP success rate. But we > shouldn't conflate it with the data corruption bug fixed in the hunks > below: > >> } else if (trylock_page(page)) { >> get_page(page); >> xas_unlock_irq(&xas); >> @@ -1626,7 +1642,12 @@ static void collapse_file(struct mm_stru >> * without racing with truncate. >> */ >> VM_BUG_ON_PAGE(!PageLocked(page), page); >> - VM_BUG_ON_PAGE(!PageUptodate(page), page); >> + >> + /* double check the page is up to date */ >> + if (unlikely(!PageUptodate(page))) { >> + result = SCAN_FAIL; >> + goto out_unlock; >> + } > > This... > >> @@ -1642,6 +1663,16 @@ static void collapse_file(struct mm_stru >> goto out_unlock; >> } >> >> + if (!is_shmem && PageDirty(page)) { >> + /* >> + * khugepaged only works on read-only fd, so this >> + * page is dirty because it hasn't been flushed >> + * since first write. >> + */ >> + result = SCAN_FAIL; >> + goto out_unlock; >> + } > > ...and this are pretty urgent chunks because they protect the > integrity of the page cache. They should go in asap. > > So I suggest we do two patches instead: Here are the two patches: > > 1. These two locked page checks to fix the corruption in 5.4 >From 6836c7c91396c7b661b2447609ec236badba62b0 Mon Sep 17 00:00:00 2001 From: Song Liu <songliubraving@xxxxxx> Date: Wed, 23 Oct 2019 11:24:28 +1100 Subject: [PATCH 1/2] mm,thp: recheck each page before collapsing file THP In collapse_file(), for !is_shmem case, current check cannot guarantee the locked page is up-to-date. Specifically, xas_unlock_irq() should not be called before lock_page() and get_page(); and it is necessary to recheck PageUptodate() after locking the page. With this bug and CONFIG_READ_ONLY_THP_FOR_FS=y, madvise(HUGE)'ed .text may contain corrupted data. This is because khugepaged mistakenly collapses some not up-to-date sub pages into a huge page, and assumes the huge page is up-to-date. This will NOT corrupt data in the disk, because the page is read-only and never written back. Fix this by properly checking PageUptodate() after locking the page. This check replaces "VM_BUG_ON_PAGE(!PageUptodate(page), page);". Also, move PageDirty() check after locking the page. Current khugepaged should not try to collapse dirty file THP, because it is limited to read-only .text. The only case we hit a dirty page here is when the page hasn't been written since write. Bail out and retry when this happens. syzbot reported bug on previous version of this patch. [songliubraving@xxxxxx: v4] Link: http://lkml.kernel.org/r/20191022191006.411277-1-songliubraving@xxxxxx [songliubraving@xxxxxx: fix deadlock in collapse_file()] Link: http://lkml.kernel.org/r/20191028221414.3685035-1-songliubraving@xxxxxx Link: http://lkml.kernel.org/r/20191018180345.4188310-1-songliubraving@xxxxxx [songliubraving@xxxxxx: flush file for !is_shmem PageDirty() case in collapse_file()] https://lkml.kernel.org/linux-mm/20191030200736.3455046-1-songliubraving@xxxxxx/ Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS") Reported-by: syzbot+efb9e48b9fbdc49bb34a@xxxxxxxxxxxxxxxxxxxxxxxxx Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: William Kucharski <william.kucharski@xxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Song Liu <songliubraving@xxxxxx> --- mm/khugepaged.c | 28 ++++++++++++++++------------ 1 file changed, 16 insertions(+), 12 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 0a1b4b484ac5..40215795d641 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1601,17 +1601,6 @@ static void collapse_file(struct mm_struct *mm, result = SCAN_FAIL; goto xa_unlocked; } - } else if (!PageUptodate(page)) { - xas_unlock_irq(&xas); - wait_on_page_locked(page); - if (!trylock_page(page)) { - result = SCAN_PAGE_LOCK; - goto xa_unlocked; - } - get_page(page); - } else if (PageDirty(page)) { - result = SCAN_FAIL; - goto xa_locked; } else if (trylock_page(page)) { get_page(page); xas_unlock_irq(&xas); @@ -1626,7 +1615,12 @@ static void collapse_file(struct mm_struct *mm, * without racing with truncate. */ VM_BUG_ON_PAGE(!PageLocked(page), page); - VM_BUG_ON_PAGE(!PageUptodate(page), page); + + /* make sure the page is up to date */ + if (unlikely(!PageUptodate(page))) { + result = SCAN_FAIL; + goto out_unlock; + } /* * If file was truncated then extended, or hole-punched, before @@ -1642,6 +1636,16 @@ static void collapse_file(struct mm_struct *mm, goto out_unlock; } + if (!is_shmem && PageDirty(page)) { + /* + * khugepaged only works on read-only fd, so this + * page is dirty because it hasn't been flushed + * since first write. + */ + result = SCAN_FAIL; + goto out_unlock; + } + if (isolate_lru_page(page)) { result = SCAN_DEL_PAGE_LRU; goto out_unlock; -- 2.17.1 > 2. The filemap_flush() bit to improve THP coverage in 5.5 >From 70092bc81f255314c1d3b1ebb7718ae2be569495 Mon Sep 17 00:00:00 2001 From: Song Liu <songliubraving@xxxxxx> Date: Tue, 29 Oct 2019 10:08:48 -0700 Subject: [PATCH 2/2] mm/thp: flush file for !is_shmem PageDirty() case in collapse_file() For non-shmem file THPs, khugepaged only collapses read only .text mapping (VM_DENYWRITE). These pages should not be dirty except the case where the file hasn't been flushed since first write. Call filemap_flush() in collapse_file() to accelerate the write back in such cases. [songliubraving@xxxxxx: flush file for !is_shmem PageDirty() case in collapse_file()] https://lkml.kernel.org/linux-mm/20191030200736.3455046-1-songliubraving@xxxxxx/ Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: William Kucharski <william.kucharski@xxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Signed-off-by: Song Liu <songliubraving@xxxxxx> --- mm/khugepaged.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 40215795d641..513cce897794 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1601,6 +1601,24 @@ static void collapse_file(struct mm_struct *mm, result = SCAN_FAIL; goto xa_unlocked; } + } else if (PageDirty(page)) { + /* + * khugepaged only works on read-only fd, + * so this page is dirty because it hasn't + * been flushed since first write. There + * won't be new dirty pages. + * + * Trigger async flush here and hope the + * writeback is done when khugepaged + * revisits this page. + * + * This is a one-off situation. We are not + * forcing writeback in loop. + */ + xas_unlock_irq(&xas); + filemap_flush(mapping); + result = SCAN_FAIL; + goto xa_unlocked; } else if (trylock_page(page)) { get_page(page); xas_unlock_irq(&xas); -- 2.17.1
