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SubjectRe: [PATCH 0/6] shmem: high order folios support in write path
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On 15.09.23 17:34, Matthew Wilcox wrote:
> On Fri, Sep 15, 2023 at 05:29:51PM +0200, David Hildenbrand wrote:
>> On 15.09.23 11:51, Daniel Gomez wrote:
>>> This series add support for high order folios in shmem write
>>> path.
>>> There are at least 2 cases/topics to handle that I'd appreciate
>>> feedback.
>>> 1. With the new strategy, you might end up with a folio order matching
>>> HPAGE_PMD_ORDER. However, we won't respect the 'huge' flag anymore if
>>> THP is enabled.
>>> 2. When the above (1.) occurs, the code skips the huge path, so
>>> xa_find with hindex is skipped.
>>
>> Similar to large anon folios (but different to large non-shmem folios in the
>> pagecache), this can result in memory waste.
>
> No, it can't. This patchset triggers only on write, not on read or page
> fault, and it's conservative, so it will only allocate folios which are
> entirely covered by the write. IOW this is memory we must allocate in
> order to satisfy the write; we're just allocating it in larger chunks
> when we can.

Oh, good! I was assuming you would eventually over-allocate on the write
path.

--
Cheers,

David / dhildenb

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