Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:6 as permitted sender) client-ip=2620:137:e000::3:6;
Message-ID: <a5c37d6e-ca0f-65cf-a264-d1220d3c3c6d@redhat.com>
Date:   Fri, 15 Sep 2023 17:36:27 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.13.0
Subject: Re: [PATCH 0/6] shmem: high order folios support in write path
Content-Language: en-US
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Daniel Gomez <da.gomez@samsung.com>,
        "minchan@kernel.org" <minchan@kernel.org>,
        "senozhatsky@chromium.org" <senozhatsky@chromium.org>,
        "axboe@kernel.dk" <axboe@kernel.dk>,
        "djwong@kernel.org" <djwong@kernel.org>,
        "hughd@google.com" <hughd@google.com>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "mcgrof@kernel.org" <mcgrof@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "gost.dev@samsung.com" <gost.dev@samsung.com>,
        Pankaj Raghav <p.raghav@samsung.com>
References: <CGME20230915095123eucas1p2c23d8a8d910f5a8e9fd077dd9579ad0a@eucas1p2.samsung.com>
 <20230915095042.1320180-1-da.gomez@samsung.com>
 <b8f75b8e-77f5-4aa1-ce73-6c90f7d87d43@redhat.com>
 <ZQR5nq7mKBJKEFHL@casper.infradead.org>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <ZQR5nq7mKBJKEFHL@casper.infradead.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 15.09.23 17:34, Matthew Wilcox wrote:
> 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.

Oh, good! I was assuming you would eventually over-allocate on the write 
path.

-- 
Cheers,

David / dhildenb