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Date:   Wed, 26 Feb 2020 17:47:16 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Matthew Wilcox <willy@infradead.org>
cc:     Yang Shi <yang.shi@linux.alibaba.com>,
        Hugh Dickins <hughd@google.com>,
        kirill.shutemov@linux.intel.com, aarcange@redhat.com,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [v2 PATCH] mm: shmem: allow split THP when truncating THP
 partially
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On Wed, 26 Feb 2020, Matthew Wilcox wrote:
> 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.

You're right to be moving in that direction, but yes, that is a larger
task, and I think both Yang and I have to decline your awful gratitude :)

Hugh