Re: [PATCH 0/6] shmem: high order folios support in write path

[David Hildenbrand]

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