Many thanks to Kirill for reminding that PageDoubleMap cannot be relied on to warn of pte mappings in the Anon THP case; and a scan of subpages does not seem appropriate here. Note how follow_trans_huge_pmd() does not even mark an Anon THP as mlocked when compound_mapcount != 1: multiple mlocking of Anon THP is avoided, so simply return from page_mlock() in this case. Link: https://lore.kernel.org/lkml/cfa154c-d595-406-eb7d-eb9df730f944@xxxxxxxxxx/ Fixes: d9770fcc1c0c ("mm/rmap: fix old bug: munlocking THP missed other mlocks") Reported-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Alistair Popple <apopple@xxxxxxxxxx> Cc: Jason Gunthorpe <jgg@xxxxxxxxxx> Cc: Ralph Campbell <rcampbell@xxxxxxxxxx> Cc: Christoph Hellwig <hch@xxxxxx> Cc: Yang Shi <shy828301@xxxxxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> --- Linus, thanks a lot for last-minute hoovering up those four mm/rmap patches, with 3/4 fixing the syzbot and 0day reports on munlocking ... BUT ... the version of 2/4 in 5.14-rc1 is defective (PageDoubleMap is a confusing flag which behaves differently on anon and file), Kirill had spotted that, and what he Acked was the v2 which went into mmotm, rather than the first version I posted. This patch here converts the v1 in rc1 into the v2 Acked by Kirill. What will go wrong with v1 in? I don't actually know: nothing terrible, can only affect people splitting and mlocking anon THPs, maybe nobody and nobot will notice, maybe some VM_BUG_ONs or "Bad page"s will turn up. I'll be on the lookout to point reporters to this fix (more lines than strictly necessary, because it removes a level of indentation). And sorry for putting 2/4 before the more urgent 3/4, but I couldn't tell what to do in 3/4, without first fixing the older bug in 2/4. Hugh mm/rmap.c | 39 ++++++++++++++++++++++----------------- 1 file changed, 22 insertions(+), 17 deletions(-) --- 5.14-rc1/mm/rmap.c +++ linux/mm/rmap.c @@ -1440,21 +1440,20 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, /* * If the page is mlock()d, we cannot swap it out. */ - if (!(flags & TTU_IGNORE_MLOCK)) { - if (vma->vm_flags & VM_LOCKED) { - /* PTE-mapped THP are never marked as mlocked */ - if (!PageTransCompound(page) || - (PageHead(page) && !PageDoubleMap(page))) { - /* - * Holding pte lock, we do *not* need - * mmap_lock here - */ - mlock_vma_page(page); - } - ret = false; - page_vma_mapped_walk_done(&pvmw); - break; - } + if (!(flags & TTU_IGNORE_MLOCK) && + (vma->vm_flags & VM_LOCKED)) { + /* + * PTE-mapped THP are never marked as mlocked: so do + * not set it on a DoubleMap THP, nor on an Anon THP + * (which may still be PTE-mapped after DoubleMap was + * cleared). But stop unmapping even in those cases. + */ + if (!PageTransCompound(page) || (PageHead(page) && + !PageDoubleMap(page) && !PageAnon(page))) + mlock_vma_page(page); + page_vma_mapped_walk_done(&pvmw); + ret = false; + break; } /* Unexpected PMD-mapped THP? */ @@ -1986,8 +1985,10 @@ static bool page_mlock_one(struct page *page, struct vm_area_struct *vma, */ if (vma->vm_flags & VM_LOCKED) { /* - * PTE-mapped THP are never marked as mlocked, but - * this function is never called when PageDoubleMap(). + * PTE-mapped THP are never marked as mlocked; but + * this function is never called on a DoubleMap THP, + * nor on an Anon THP (which may still be PTE-mapped + * after DoubleMap was cleared). */ mlock_vma_page(page); /* @@ -2022,6 +2023,10 @@ void page_mlock(struct page *page) VM_BUG_ON_PAGE(!PageLocked(page) || PageLRU(page), page); VM_BUG_ON_PAGE(PageCompound(page) && PageDoubleMap(page), page); + /* Anon THP are only marked as mlocked when singly mapped */ + if (PageTransCompound(page) && PageAnon(page)) + return; + rmap_walk(page, &rwc); }
