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SubjectRe: [PATCH] mm/slub: replace alloc_pages with folio_alloc
On Sun, May 29, 2022 at 04:31:07AM +0100, Matthew Wilcox wrote:
> On Sun, May 29, 2022 at 10:58:18AM +0800, Muchun Song wrote:
> > On Sat, May 28, 2022 at 05:27:11PM +0100, Matthew Wilcox wrote:
> > > On Sun, May 29, 2022 at 12:11:58AM +0800, bh1scw@gmail.com wrote:
> > > > From: Fanjun Kong <bh1scw@gmail.com>
> > > >
> > > > This patch will use folio allocation functions for allocating pages.
> > >
> > > That's not actually a good idea. folio_alloc() will do the
> > > prep_transhuge_page() step which isn't needed for slab.
> >
> > You mean folio_alloc() is dedicated for THP allocation? It is a little
> > surprise to me. I thought folio_alloc() is just a variant of alloc_page(),
> > which returns a folio struct instead of a page. Seems like I was wrong.
> > May I ask what made us decide to do this?
>
> Yeah, the naming isn't great here. The problem didn't really occur
> to me until I saw this patch, and I don't have a good solution yet.

OK, I have an idea.

None of the sl*b allocators use the page refcount. So the
atomic operations on it are just a waste of time. If we add an
alloc_unref_page() to match our free_unref_page(), that'll be enough
difference to stop pepole sending "helpful" patches. Also, it'll be a
(small?) performance improvement for slab.

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