linux-kernel - Re: [PATCH 0/6] shmem: high order folios support in write path

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Date:   Fri, 15 Sep 2023 16:34:54 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Daniel Gomez <da.gomez@...sung.com>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "senozhatsky@...omium.org" <senozhatsky@...omium.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "djwong@...nel.org" <djwong@...nel.org>,
        "hughd@...gle.com" <hughd@...gle.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mcgrof@...nel.org" <mcgrof@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "gost.dev@...sung.com" <gost.dev@...sung.com>,
        Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH 0/6] shmem: high order folios support in write path

On Fri, Sep 15, 2023 at 05:29:51PM +0200, David Hildenbrand wrote:
> On 15.09.23 11:51, Daniel Gomez wrote:
> > This series add support for high order folios in shmem write
> > path.
> > There are at least 2 cases/topics to handle that I'd appreciate
> > feedback.
> > 1. With the new strategy, you might end up with a folio order matching
> > HPAGE_PMD_ORDER. However, we won't respect the 'huge' flag anymore if
> > THP is enabled.
> > 2. When the above (1.) occurs, the code skips the huge path, so
> > xa_find with hindex is skipped.
> 
> Similar to large anon folios (but different to large non-shmem folios in the
> pagecache), this can result in memory waste.

No, it can't.  This patchset triggers only on write, not on read or page
fault, and it's conservative, so it will only allocate folios which are
entirely covered by the write.  IOW this is memory we must allocate in
order to satisfy the write; we're just allocating it in larger chunks
when we can.