On Mon, 4 Oct 2021, Matthew Wilcox wrote: > On Mon, Oct 04, 2021 at 11:28:45AM -0700, Yang Shi wrote: > > On Sat, Oct 2, 2021 at 10:09 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > On Thu, Sep 30, 2021 at 10:39:14AM -0700, Yang Shi wrote: > > > > On Thu, Sep 30, 2021 at 9:49 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > > > > I assume you're thinking of one of the fuzzer blkdev ones: > > > > > https://lore.kernel.org/linux-mm/CACkBjsbtF_peC7N_4mRfHML_BeiPe+O9DahTfr84puSG_J9rcQ@xxxxxxxxxxxxxx/ > > > > > or > > > > > https://lore.kernel.org/lkml/CACkBjsYwLYLRmX8GpsDpMthagWOjWWrNxqY6ZLNQVr6yx+f5vA@xxxxxxxxxxxxxx/ > > > > > > > > > > I haven't started on those ones yet: yes, I imagine one or both of those > > > > > will need a further fix (S_ISREG() check somewhere if we're lucky; but > > > > > could well be nastier); but for the bug in this thread, I expect > > > > > > > > Makes sense to me. We should be able to check S_ISREG() in khugepaged, > > > > if it is not a regular file, just bail out. Sounds not that nasty to > > > > me AFAIU. > > > > > > I don't see why we should have an S_ISREG() check. I agree it's not the > > > intended usecase, but it ought to work fine. Unless there's something > > > I'm missing? > > > > Check out this bug report: > > https://lore.kernel.org/lkml/CACkBjsYwLYLRmX8GpsDpMthagWOjWWrNxqY6ZLNQVr6yx+f5vA@xxxxxxxxxxxxxx/ > > and the patch from me: > > https://lore.kernel.org/linux-mm/20210917205731.262693-1-shy828301@xxxxxxxxx/ > > > > I don't think we handle buffers correctly for file THP, right? My > > patch is ad hoc, so I thought Hugh's suggestion makes some sense to > > me. Why do we have THP collapsed for unintended usecase in the first > > place? > > OK, I've done some more digging. I think what's going on with this > report is userspace opens the block device RO, causes the page cache to > be loaded with data, then khugepaged comes in and creates THPs. Yes. > What confuses me is that these THPs have private data attached to them. > I don't know how that happens. If it's block device specific, then > yes, something like your S_ISREG() idea should work fine. Otherwise, > we might need to track down another problem. Agreed, the file THP is created without PagePrivate, so the puzzle was why the read-only cached page would later become page_has_private(). The C repro showed that it uses (a BTRFS_IOC_ADD_DEV ioctl which might not be relevant and) a BLKRRPART ioctl 0x125f: I didn't follow BLKRRPART all the way down, but imagine it has to attach buffer-heads to re-read the partition table. Which would explain it. Aside from that particular ioctl, it seems a good idea to insist on S_ISREG just to shrink the attack surface: as Yang Shi says, executable THP on block device was never an intended usecase, and not a usecase anyone is likely to miss! And that fuzzer appears to delight in tormenting /dev/nullb0, so let's just seal off that avenue. You're right to have some doubt, as to whether there might be other ways for buffer-heads to get attached, even on a read-only regular file; but no way has sprung to my mind, and READ_ONLY_THP_FOR_FS has survived well in its intended usage: so I think we should proceed on the assumption that no further bugs remain - then fix them when found. I wasn't able to reproduce the problem with the repro, would need to waste many hours to do so. But here's the untested S_ISREG patch I came up with. Sorry, I've mixed something else in: in moving the alignment part to clarify the conditions, I was alarmed to see that shmem with !shmem_huge_enabled was falling through to THP_FOR_FS to give unexpected huge pages: fixed that, though later found there's a separate shmem_huge_enabled() check which should exclude it. --- 5.15-rc4/mm/khugepaged.c 2021-09-12 17:39:21.943438422 -0700 +++ linux/khugepaged.c 2021-10-03 20:41:13.194822795 -0700 @@ -445,22 +445,25 @@ static bool hugepage_vma_check(struct vm if (!transhuge_vma_enabled(vma, vm_flags)) return false; + if (vma->vm_file && !IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - + vma->vm_pgoff, HPAGE_PMD_NR)) + return false; + /* Enabled via shmem mount options or sysfs settings. */ - if (shmem_file(vma->vm_file) && shmem_huge_enabled(vma)) { - return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff, - HPAGE_PMD_NR); - } + if (shmem_file(vma->vm_file)) + return shmem_huge_enabled(vma); /* THP settings require madvise. */ if (!(vm_flags & VM_HUGEPAGE) && !khugepaged_always()) return false; /* Read-only file mappings need to be aligned for THP to work. */ - if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && vma->vm_file && - !inode_is_open_for_write(vma->vm_file->f_inode) && - (vm_flags & VM_EXEC)) { - return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff, - HPAGE_PMD_NR); + if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && + (vm_flags & VM_EXEC) && vma->vm_file) { + struct inode *inode = vma->vm_file->f_inode; + + return !inode_is_open_for_write(inode) && + S_ISREG(inode->i_mode); } if (!vma->anon_vma || vma->vm_ops)
