linux-kernel - Re: [PATCH mm-unstable v2 1/6] mm: Add folio_estimated

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Message-ID: <bbd6857a-1303-34ac-9d1b-f61a540ea796@redhat.com>
Date:   Mon, 30 Jan 2023 10:34:03 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jane Chu <jane.chu@...cle.com>,
        Vishal Moola <vishal.moola@...il.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable v2 1/6] mm: Add folio_estimated_mapcount()

On 28.01.23 01:48, Jane Chu wrote:
> On 1/26/2023 12:37 AM, David Hildenbrand wrote:
>> On 25.01.23 23:09, Vishal Moola wrote:
> [..]
>>
>> The issue is that we're not estimating the mapcount of the folio, so the
>> name is very misleading ... I think you really want to avoid the term
>> mapcount completely in that context. We're just using the #mappings of
>> the first subpage to determine something differently.
>>
>> Thinking about it, I guess "folio_estimated_sharers()" might be what we
>> actually want to name it. Then you can comment how we estimate sharers
>> by looking at into how many page tables the first subpage is currently
>> mapped, and assume the same holds true for the other subpages.
>>
>> It's unreliable because other subpages might behave differently, we
>> might not be holding the pagelock to stabilize, and we're not looking at
>> indirect mappings via the swapcache. But it's a comapratively good
>> estimate for most scenarios I guess.
>>
> 
> Hmm, how about simply call it folio_hpage_mapcount(),
> folio_firstpage_mapcount(), or, folio_cover_mapcount() ?

All not better IMHO.

folio_estimated_subpage_mapcount() is a bit too verbose for my taste and ...

> 
> It is used to replace page_mapcount() in that sense -
> https://lore.kernel.org/linux-mm/Y9MDJuPWsk9820xD@x1n/T/#me0531cfb9e82ad5ca88804c727d69cc6b9b33ffa
> 
> 	if (flags & (MPOL_MF_MOVE_ALL) ||
> 	    (flags & MPOL_MF_MOVE && folio_estimated_mapcount(folio) == 1 &&
> 	     !hugetlb_pmd_shared(pte))) {
> 		if (isolate_hugetlb(folio, qp->pagelist) &&

... what we want to have here is an estimation on the number of sharers. 
[actually, we would want it precise, but that's hard to achieve ... ]

-- 
Thanks,

David / dhildenb