linux-kernel - Re: [PATCH 04/12] mm/vmscan: Free non-shmem folios without splitting them

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Message-ID: <20220117210057.o2aug2unmovufrdz@box.shutemov.name>
Date:   Tue, 18 Jan 2022 00:00:57 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 04/12] mm/vmscan: Free non-shmem folios without splitting
 them

On Mon, Jan 17, 2022 at 04:10:46PM +0000, Matthew Wilcox wrote:
> On Mon, Jan 17, 2022 at 07:06:25PM +0300, Kirill A. Shutemov wrote:
> > On Sun, Jan 16, 2022 at 12:18:14PM +0000, Matthew Wilcox (Oracle) wrote:
> > > We have to allocate memory in order to split a file-backed folio, so
> > > it's not a good idea to split them in the memory freeing path.
> > 
> > Could elaborate on why split a file-backed folio requires memory
> > allocation?
> 
> In the commit message or explain it to you now?
> 
> We need to allocate xarray nodes to store all the newly-independent
> pages.  With a folio that's more than 64 entries in size (current
> implementation), we elide the lowest layer of the radix tree.  But
> with any data structure that tracks folios, we'll need to create
> space in it to track N folios instead of 1.

Looks good.

> > > It also
> > > doesn't work for XFS because pages have an extra reference count from
> > > page_has_private() and split_huge_page() expects that reference to have
> > > already been removed.
> > 
> > Need to adjust can_split_huge_page()?
> 
> no?

I meant we can make can_split_huge_page() expect extra pin if
page_has_private() is true. If it is the only thing that stops
split_huge_page() from handling XFS pages.

-- 
 Kirill A. Shutemov