linux-kernel - Re: [PATCH v2 2/3] mm: Implement folio_remove_rmap

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Date:   Wed, 19 Jul 2023 19:46:46 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 2/3] mm: Implement folio_remove_rmap_range()

On 19/07/2023 19:23, Yu Zhao wrote:
> On Wed, Jul 19, 2023 at 7:55 AM Ryan Roberts <ryan.roberts@....com> wrote:
>>
>> Like page_remove_rmap() but batch-removes the rmap for a range of pages
>> belonging to a folio. This can provide a small speedup due to less
>> manipuation of the various counters. But more crucially, if removing the
>> rmap for all pages of a folio in a batch, there is no need to
>> (spuriously) add it to the deferred split list, which saves significant
>> cost when there is contention for the split queue lock.
>>
>> All contained pages are accounted using the order-0 folio (or base page)
>> scheme.
>>
>> Reviewed-by: Yin Fengwei <fengwei.yin@...el.com>
>> Reviewed-by: Zi Yan <ziy@...dia.com>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> 
> I have asked for this before but let me be very clear this time: we
> need to generalize the existing functions rather than add more
> specialized functions. Otherwise it'd get even harder to maintain down
> the road.

Yeah fair enough, my fault; I wrote this before I had your feedback on the other
rmap function and overlooked it when refactoring this. I'll fix it and repost.

> 
> folio_remove_rmap_range() needs to replace page_remove_rmap(). IOW,
> page_remove_rmap() is just a wrapper around folio_remove_rmap_range().