On 3/30/22 00:58, David Hildenbrand wrote:
> On 24.03.22 18:06, Mike Kravetz wrote:
>> hugetlbfs fallocate support was originally added with commit 70c3547e36f5
>> ("hugetlbfs: add hugetlbfs_fallocate()"). Initial support only operated
>> on whole hugetlb pages. This makes sense for populating files as other
>> interfaces such as mmap and truncate require hugetlb page size alignment.
>> Only operating on whole hugetlb pages for the hole punch case was a
>> simplification and there was no compelling use case to zero partial pages.
>>
>> In a recent discussion[1] it was assumed that hugetlbfs hole punch would
>> zero partial hugetlb pages as that is in line with the man page
>> description saying 'partial filesystem blocks are zeroed'. However,
>> the hugetlbfs hole punch code actually does this:
>>
>> hole_start = round_up(offset, hpage_size);
>> hole_end = round_down(offset + len, hpage_size);
>>
>> Modify code to zero partial hugetlb pages in hole punch range. It is
>> possible that application code could note a change in behavior. However,
>> that would imply the code is passing in an unaligned range and expecting
>> only whole pages be removed. This is unlikely as the fallocate
>> documentation states the opposite.
>
> Yeah, some weird code could behave differently; an app would have to
> pass in an unaligned range and expect that partially covered hugetlbfs
> pages remain unmodified. It's hard to think of reasonable apps that do
> that, but of course, some buggy code might then be *actually* broken.
> Like some messed-up align-up implementation that accidentally adds +1
> too much.
>
>>
>> The current hugetlbfs fallocate hole punch behavior is tested with the
>> libhugetlbfs test fallocate_align[2]. This test will be updated to
>> validate partial page zeroing.
>
> This is in line with other fallocate() behavior and documented
> semantics, so I think that's the right thing to do.
>
> I think it's worth to give it a try, it's hard to imagine that this
> actually breaks something.
>
>
> "After a successful call, subsequent reads from this range will return
> zeros." will work as expected with your change.
>
>>
>> [1] https://lore.kernel.org/linux-mm/20571829-9d3d-0b48-817c-b6b15565f651@xxxxxxxxxx/
>> [2] https://github.com/libhugetlbfs/libhugetlbfs/blob/master/tests/fallocate_align.c
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>> ---
>> fs/hugetlbfs/inode.c | 67 ++++++++++++++++++++++++++++++---------
>> include/asm-generic/tlb.h | 2 ++
>> 2 files changed, 54 insertions(+), 15 deletions(-)
>>
>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>> index a7c6c7498be0..f62ec4f71132 100644
>> --- a/fs/hugetlbfs/inode.c
>> +++ b/fs/hugetlbfs/inode.c
>> @@ -587,41 +587,78 @@ static void hugetlb_vmtruncate(struct inode *inode, loff_t offset)
>> remove_inode_hugepages(inode, offset, LLONG_MAX);
>> }
>>
>> +static void hugetlbfs_zero_partial_page(struct hstate *h,
>> + struct address_space *mapping,
>> + unsigned long start,
>> + unsigned long end)
>> +{
>> + struct page *page;
>> + pgoff_t idx = start >> huge_page_shift(h);
>
> I'm a fan of reverse Christmas trees :)
>
Ok, no preference by me so I will change.
...
>> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
>> index 2c68a545ffa7..4622ee45f739 100644
>> --- a/include/asm-generic/tlb.h
>> +++ b/include/asm-generic/tlb.h
>> @@ -562,6 +562,7 @@ static inline void tlb_flush_p4d_range(struct mmu_gather *tlb,
>> __tlb_remove_tlb_entry(tlb, ptep, address); \
>> } while (0)
>>
>> +#ifndef tlb_remove_huge_tlb_entry
>> #define tlb_remove_huge_tlb_entry(h, tlb, ptep, address) \
>> do { \
>> unsigned long _sz = huge_page_size(h); \
>> @@ -571,6 +572,7 @@ static inline void tlb_flush_p4d_range(struct mmu_gather *tlb,
>> tlb_flush_pud_range(tlb, address, _sz); \
>> __tlb_remove_tlb_entry(tlb, ptep, address); \
>> } while (0)
>> +#endif
>
> Was this change supposed to be included in this patch?
No, this should not have been included. Sorry.
Thanks for taking a look.
--
Mike Kravetz
