Re: [PATCH mm-unstable v2 1/6] mm: Add folio_estimated_mapcount()

From: Jane Chu <jane.chu@oracle.com>
To: David Hildenbrand <david@redhat.com>,
	Vishal Moola <vishal.moola@gmail.com>
Cc: linux-mm@kvack.org, akpm@linux-foundation.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH mm-unstable v2 1/6] mm: Add folio_estimated_mapcount()
Date: Fri, 27 Jan 2023 16:48:23 -0800	[thread overview]
Message-ID: <3a6af06a-729d-7659-50b0-8b242b555134@oracle.com> (raw)
In-Reply-To: <52af321f-175b-9fa9-10f0-ac2bba04c677@redhat.com>

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() ?

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) &&

thanks,
-jane