linux-kernel - Re: [PATCH v3 2/3] mm: Implement folio_remove_rmap

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Date:   Thu, 27 Jul 2023 08:26:44 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Matthew Wilcox <willy@...radead.org>,
        "Huang, Ying" <ying.huang@...el.com>
Cc:     Yu Zhao <yuzhao@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 2/3] mm: Implement folio_remove_rmap_range()

On 27/07/2023 03:35, Matthew Wilcox wrote:
> On Thu, Jul 27, 2023 at 09:29:24AM +0800, Huang, Ying wrote:
>> Matthew Wilcox <willy@...radead.org> writes:
>>> I think that can make sense.  Because we limit to a single page table,
>>> specifying 'nr = 1 << PMD_ORDER' is the same as 'compound = true'.
>>> Just make it folio, page, nr, vma.  I'd actually prefer it as (vma,
>>> folio, page, nr), but that isn't the convention we've had in rmap up
>>> until now.
>>
>> IIUC, even if 'nr = 1 << PMD_ORDER', we may remove one PMD 'compound'
>> mapping, or 'nr' PTE mapping.  So, we will still need 'compound' (or
>> some better name) as parameter.
> 
> Oh, this is removing ... so you're concerned with the case where we've
> split the PMD into PTEs, but all the PTEs are still present in a single
> page table?  OK, I don't have a good answer to that.  Maybe that torpedoes
> the whole idea; I'll think about it.

This is exactly why I think the approach I've already taken is the correct one;
a 'range' makes no sense when you are dealing with 'compound' pages because you
are accounting the entire folio. So surely its better to reflect that by only
accounting small pages in the range version of the API.