file_thp_enabled() incorrectly allows THP for files on anonymous inodes (e.g. guest_memfd and secretmem). These files are created via alloc_file_pseudo(), which does not call get_write_access() and leaves inode->i_writecount at 0. Combined with S_ISREG(inode->i_mode) being true, they appear as read-only regular files when CONFIG_READ_ONLY_THP_FOR_FS is enabled, making them eligible for THP collapse. Anonymous inodes can never pass the inode_is_open_for_write() check since their i_writecount is never incremented through the normal VFS open path. The right thing to do is to exclude them from THP eligibility altogether, since CONFIG_READ_ONLY_THP_FOR_FS was designed for real filesystem files (e.g. shared libraries), not for pseudo-filesystem inodes. For guest_memfd, this allows khugepaged and MADV_COLLAPSE to create large folios in the page cache via the collapse path, but the guest_memfd fault handler does not support large folios. This triggers WARN_ON_ONCE(folio_test_large(folio)) in kvm_gmem_fault_user_mapping(). For secretmem, collapse_file() tries to copy page contents through the direct map, but secretmem pages are removed from the direct map. This can result in a kernel crash: BUG: unable to handle page fault for address: ffff88810284d000 RIP: 0010:memcpy_orig+0x16/0x130 Call Trace: collapse_file hpage_collapse_scan_file madvise_collapse Secretmem is not affected by the crash on upstream as the memory failure recovery handles the failed copy gracefully, but it still triggers confusing false memory failure reports: Memory failure: 0x106d96f: recovery action for clean unevictable LRU page: Recovered Check IS_ANON_FILE(inode) in file_thp_enabled() to deny THP for all anonymous inode files. Link: https://syzkaller.appspot.com/bug?extid=33a04338019ac7e43a44 Link: https://lore.kernel.org/linux-mm/CAEvNRgHegcz3ro35ixkDw39ES8=U6rs6S7iP0gkR9enr7HoGtA@mail.gmail.com Reported-by: syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=33a04338019ac7e43a44 Fixes: 7fbb5e188248 ("mm: remove VM_EXEC requirement for THP eligibility") Tested-by: syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com Cc: stable@vger.kernel.org Signed-off-by: Deepanshu Kartikey --- v2: - Use IS_ANON_FILE(inode) to deny THP for all anonymous inode files instead of checking for specific subsystems (David Hildenbrand) - Updated Fixes tag to 7fbb5e188248 which removed the VM_EXEC requirement that accidentally protected secretmem - Expanded commit message with implications for both guest_memfd and secretmem --- mm/huge_memory.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 40cf59301c21..d3beddd8cc30 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -94,6 +94,9 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma) inode = file_inode(vma->vm_file); + if (IS_ANON_FILE(inode)) + return false; + return !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode); } -- 2.43.0