Re: [PATCH v3] Fix ext4 fault handling when mounted with -o dax,ro

[Randy Dodgen]

An xfstest is a fine idea. I've started some work on that.

On Tue, Aug 29, 2017 at 2:37 PM, Ross Zwisler
 wrote:
> On Tue, Aug 29, 2017 at 02:20:02PM -0700, Christoph Hellwig wrote:
>> Randy, any chance yto at least share a test script so that others
>> can wire it up for the test suite?
>
> I made a reproducer for my testing. I'll make an xfstest if Randy isn't able
> to.
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Re: [PATCH v2] Fix ext4 fault handling when mounted with -o dax,ro

[Randy Dodgen]

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
<ross.zwis...@linux.intel.com> wrote:
> On Tue, Aug 22, 2017 at 08:37:04PM -0700, rdod...@gmail.com wrote:
>> From: Randy Dodgen <dod...@google.com>
>>
>> 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 <dod...@google.com>
>> ---
>>
>> 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
>