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

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Date:   Thu, 27 Jul 2023 03:35:36 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Yu Zhao <yuzhao@...gle.com>, Ryan Roberts <ryan.roberts@....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 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.