From: Randy Dodgen Subject: Re: [PATCH v2] Fix ext4 fault handling when mounted with -o dax,ro Date: Wed, 23 Aug 2017 13:11:21 -0700 Message-ID: References: <20170822222434.146233-1-dodgen@google.com> <20170823163826.GB25999@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Cc: "Theodore Ts'o" , linux-ext4@vger.kernel.org, Randy Dodgen , linux-nvdimm@lists.01.org To: Ross Zwisler Return-path: Received: from mail-qt0-f182.google.com ([209.85.216.182]:34848 "EHLO mail-qt0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932443AbdHWULX (ORCPT ); Wed, 23 Aug 2017 16:11:23 -0400 Received: by mail-qt0-f182.google.com with SMTP id x36so6314446qtx.2 for ; Wed, 23 Aug 2017 13:11:22 -0700 (PDT) In-Reply-To: <20170823163826.GB25999@linux.intel.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: That's a nice simplification. I started cautiously by replicating the same checks for dax.c (dax_iomap_pte_fault checks for cow_page specifically). I recall that it used to be possible for COW pages to appear in VM_SHARED mappings, but I'm glad to see that went away in cda540ace6a19. I'll send a new version today. One potential advantage hiding in the more complicated checks is that we avoid repeatedly grabbing the journal as we fallback from PUD -> PMD or PUD -> PMD -> PTE (see __handle_mm_fault and VM_FAULT_FALLBACK checks). I will defer to the ext4 folks w.r.t. that being worthwhile; if so, there will need to be some thought on how to tweak the new .huge_fault protocol, or how to move the journal bits after the dax_iomap_fault fallbacks (maybe in ext4_iomap_{begin, end}?) Regarding ext4's behavior in the non-DAX case, note that those vm_ops don't have a .huge_fault handler, and .fault delegates to filemap_fault (which as you mention doesn't care about FAULT_FLAG_WRITE etc). Ignoring .huge_fault, we can assume that .page_mkwrite will be called at just the right times (e.g. as part of do_shared_fault but not do_cow_fault). Meanwhile, implementing .huge_fault is much trickier; there is no ".huge_mkwrite" (so some prediction of COW is needed, as here) and one must remember to split huge entries before returning VM_FAULT_FALLBACK (see 59bf4fb9 ; not doing so in __dax_pmd_fault was resulting in repeated PMD faults not making progress). Maybe there is room to improve this. On Wed, Aug 23, 2017 at 9:38 AM, Ross Zwisler wrote: > On Tue, Aug 22, 2017 at 08:37:04PM -0700, rdodgen@gmail.com wrote: >> From: Randy Dodgen >> >> If an ext4 filesystem is mounted with both the DAX and read-only >> options, executables on that filesystem will fail to start (claiming >> 'Segmentation fault') due to the fault handler returning >> VM_FAULT_SIGBUS. >> >> This is due to the DAX fault handler (see ext4_dax_huge_fault) >> attempting to write to the journal when FAULT_FLAG_WRITE is set. This is >> the wrong behavior for write faults which will lead to a COW page; in >> particular, this fails for readonly mounts. >> >> This changes replicates some check from dax_iomap_fault to more >> precisely reason about when a journal-write is needed. >> >> It might be the case that this could be better handled in >> ext4_iomap_begin / ext4_iomap_end (called via iomap_ops inside >> dax_iomap_fault). These is some overlap already (e.g. grabbing journal >> handles). >> >> Signed-off-by: Randy Dodgen >> --- >> >> I'm resending for some DMARC-proofing (thanks Ted for the explanation), a >> missing Signed-off-by, and some extra cc's. Oops! >> >> fs/ext4/file.c | 26 +++++++++++++++++++++++++- >> 1 file changed, 25 insertions(+), 1 deletion(-) >> >> diff --git a/fs/ext4/file.c b/fs/ext4/file.c >> index 0d7cf0cc9b87..d512fb85a3e3 100644 >> --- a/fs/ext4/file.c >> +++ b/fs/ext4/file.c >> @@ -279,7 +279,31 @@ static int ext4_dax_huge_fault(struct vm_fault *vmf, >> handle_t *handle = NULL; >> struct inode *inode = file_inode(vmf->vma->vm_file); >> struct super_block *sb = inode->i_sb; >> - bool write = vmf->flags & FAULT_FLAG_WRITE; >> + bool write; >> + >> + /* >> + * We have to distinguish real writes from writes which will result in a >> + * COW page >> + * - COW writes need to fall-back to installing PTEs. See >> + * dax_iomap_pmd_fault. >> + * - COW writes should *not* poke the journal (the file will not be >> + * changed). Doing so would cause unintended failures when mounted >> + * read-only. >> + */ >> + if (pe_size == PE_SIZE_PTE) { >> + /* See dax_iomap_pte_fault. */ >> + write = (vmf->flags & FAULT_FLAG_WRITE) && !vmf->cow_page; >> + } else if (pe_size == PE_SIZE_PMD) { >> + /* See dax_iomap_pmd_fault. */ >> + write = vmf->flags & FAULT_FLAG_WRITE; >> + if (write && !(vmf->vma->vm_flags & VM_SHARED)) { >> + split_huge_pmd(vmf->vma, vmf->pmd, vmf->address); >> + count_vm_event(THP_FAULT_FALLBACK); >> + return VM_FAULT_FALLBACK; >> + } >> + } else { >> + return VM_FAULT_FALLBACK; >> + } > > This works in my setup, though the logic could be simpler. > > For all fault sizes you can rely on the fact that a COW write will happen when > we have FAULT_FLAG_WRITE but not VM_SHARED. This is the logic that we use to > know to set up vmf->cow_page() in do_fault() by calling do_cow_fault(), and in > finish_fault(). > > I think your test can then just become: > > write = (vmf->flags & FAULT_FLAG_WRITE) && > (vmf->vma->vm_flags & VM_SHARED); > > With some appropriate commenting. > > You can then let the DAX fault handlers worry about validating the fault size > and splitting the PMD on fallback. > > I'll let someone with more ext4-fu comment on whether it is okay to skip the > journal entry when doing a COW fault. This must be handled in ext4 for the > non-DAX case, but I don't see any more checks for VM_SHARED or > FAULT_FLAG_WRITE in fs/ext4, so maybe there is a better way? > > - Ross