linux-kernel - Re: [PATCH] mm: migrate: Fix THP's mapcount on isolation

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Message-ID: <ce15b4a2-b35e-bc33-9610-567438982cf3@redhat.com>
Date:   Thu, 24 Nov 2022 09:50:43 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Hugh Dickins <hughd@...gle.com>, Gavin Shan <gshan@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, william.kucharski@...cle.com,
        ziy@...dia.com, kirill.shutemov@...ux.intel.com,
        zhenyzha@...hat.com, shan.gavin@...il.com, riel@...riel.com,
        apopple@...dia.com
Subject: Re: [PATCH] mm: migrate: Fix THP's mapcount on isolation

On 23.11.22 17:07, Matthew Wilcox wrote:
> On Wed, Nov 23, 2022 at 09:56:38AM +0100, David Hildenbrand wrote:
>> But we do have an even better helper in place already:
>> mm/huge_memory.c:can_split_folio()
>>
>> Which cares about
>>
>> a) Swapcache for THP: each subpage could be in the swapcache
>> b) Requires the caller to hold one reference to be safe
>>
>> But I am a bit confused about the "extra_pins" for !anon. Where do the
>> folio_nr_pages() references come from?
> 
> When we add a folio to the page cache, we increment its refcount by
> folio_nr_pages() instead of by 1.  I suspect this is no longer needed
> (if it was ever needed) and it could be changed.  See
> __filemap_add_folio():
> 
>          long nr = 1;
>          if (!huge) {
>                  nr = folio_nr_pages(folio);
>          folio_ref_add(folio, nr);
> 
>> So *maybe* it makes sense to factor out can_split_folio() and call it
>> something like: "folio_maybe_additionally_referenced"  [to clearly
>> distinguish it from "folio_maybe_dma_pinned" that cares about actual page
>> pinning (read/write page content)].
>>
>> Such a function could return false positives/negatives due to races and the
>> caller would have to hold one reference and be able to deal with the
>> semantics.
> 
> I don't like the 'pextra_pins' parameter to a generic function ...

Right, that part should remain khugepaged specific. The assumption would 
be, that the caller of the generic function holds exactly one additional 
reference.

-- 
Thanks,

David / dhildenb