lkml.org 
[lkml]   [2022]   [Sep]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] mm: move PG_slab flag to page_type
On 25.09.22 01:04, Matthew Wilcox wrote:
> On Mon, Sep 19, 2022 at 09:57:08PM +0900, Hyeonggon Yoo wrote:
>> For now, only SLAB uses _mapcount field as a number of active objects in
>> a slab, and other slab allocators do not use it. As 16 bits are enough
>> for that, use remaining 16 bits of _mapcount as page_type even when
>> SLAB is used. And then move PG_slab flag to page_type!
>>
>> Note that page_type is always placed in upper 16 bits of _mapcount to
>> avoid confusing normal _mapcount as page_type. As underflow (actually
>> I mean, yeah, overflow) is not a concern anymore, use more lower bits
>> except bit zero.
>>
>> Add more folio helpers for PAGE_TYPE_OPS() not to break existing
>> slab implementations.
>>
>> Remove PG_slab check from PAGE_FLAGS_CHECK_AT_FREE. buddy will still
>> check if _mapcount is properly set at free.
>>
>> Exclude PG_slab from hwpoison and show_page_flags() for now.
>>
>> Note that with this patch, page_mapped() and folio_mapped() always return
>> false for slab page.
>
> This is an interesting approach. It raises some questions.
>
> First, you say that folio_mapped() returns false for slab pages. That's
> only true for order-0 slab pages. For larger pages,
>
> if (!folio_test_large(folio))
> return atomic_read(&folio->_mapcount) >= 0;
> if (atomic_read(folio_mapcount_ptr(folio)) >= 0)
> return true;
>
> so that's going to depend what folio_mapcount_ptr() aliases with.
>
> Second, this patch changes the behaviour of PageSlab() when applied to
> tail pages. Which raises the further question of what PageBuddy(),
> PageTable(), PageGuard() and PageIsolated() should do for multi-page
> folios, if that is even possible.

IIRC, these flags never apply on real compound pages so far. For
example, PageBuddy() is only set on the first page of a (budy-aligned)
free chunk of memory, and all "remaining" (tail) pages have a refcount
of zero and don't have the flag set.

There are page tables on some systems (e.g., s390x) that span multiple
pages (pmd_alloc_one()). I think the behavior is similar -- no compound
page, and only the first page has the flag set.

--
Thanks,

David / dhildenb

\
 
 \ /
  Last update: 2022-09-26 10:01    [W:0.153 / U:1.060 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site