linux-kernel - Re: [v2 PATCH] mm: shmem: allow split THP when truncating THP partially

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Message-ID: <20200227011654.GF24185@bombadil.infradead.org>
Date:   Wed, 26 Feb 2020 17:16:54 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     Hugh Dickins <hughd@...gle.com>, kirill.shutemov@...ux.intel.com,
        aarcange@...hat.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH] mm: shmem: allow split THP when truncating THP
 partially

On Wed, Feb 26, 2020 at 09:43:53AM -0800, Yang Shi wrote:
> > No.  The pagevec_lookup_entries() calls from mm/truncate.c prefer the
> > new behavior - evicting the head from page cache removes all the tails
> > along with it, so getting the tails a waste of time there too, just as
> > it was in shmem_undo_range().
> 
> TBH I'm not a fun of this hack. This would bring in other confusion or
> complexity. Pagevec is supposed to count in the number of base page, now it
> would treat THP as one page, and there might be mixed base page and THP in
> one pagevec. But, I tend to agree avoiding getting those 14 extra pins at
> the first place might be a better approach. All the complexity are used to
> release those extra pins.

My long-term goal is to eradicate tail pages entirely, so a pagevec will
end up containing pages of different sizes.  If you want to help move
in this direction, I'd be awfully grateful.  But I wouldn't say that's
in any way a prerequisite for fixing this current problem.