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Thu, 12 Dec 2024 01:16:33 -0800 (PST) MIME-Version: 1.0 References: <20241212073711.82300-1-21cnbao@gmail.com> <41e33113-1ac4-48fa-8eac-0f90ba5bc864@redhat.com> In-Reply-To: <41e33113-1ac4-48fa-8eac-0f90ba5bc864@redhat.com> From: Barry Song <21cnbao@gmail.com> Date: Thu, 12 Dec 2024 22:16:22 +1300 Message-ID: Subject: Re: [PATCH RFC] mm: map zero-filled pages to zero_pfn while doing swap-in To: David Hildenbrand Cc: Christoph Hellwig , akpm@linux-foundation.org, linux-mm@kvack.org, axboe@kernel.dk, bala.seshasayee@linux.intel.com, chrisl@kernel.org, hannes@cmpxchg.org, kanchana.p.sridhar@intel.com, kasong@tencent.com, nphamcs@gmail.com, ryan.roberts@arm.com, senozhatsky@chromium.org, terrelln@fb.com, usamaarif642@gmail.com, v-songbaohua@oppo.com, wajdi.k.feghali@intel.com, willy@infradead.org, ying.huang@linux.alibaba.com, yosryahmed@google.com, baolin.wang@linux.alibaba.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam05 X-Stat-Signature: 1crhoyekscmey6i7nep5swwszfy3yj3f X-Rspamd-Queue-Id: 90D3940009 X-Rspam-User: X-HE-Tag: 1733994983-867482 X-HE-Meta: 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 5hI60jQB Y3mRCvxoA5jH2wKMvppZ4QX5X/+ufNV9r4iypP1O/hDZ+UV0HlLPFdSmDpBa2+GRV547L3JWL1LIos/EeaT8+g0bMAI1tk5XY/ALfNCbFOr5Vo7jzTkvDLiYT85jIMZPxI8YAlKPkm1lBYD3Synag/Z3goSq4cu4pPpH3gTnAlX3Xh8GANvpqWL4xlubwQPYplXYzwXGPEFtYGWilUYzJVK0CeVnYBctQ2+Mk39hF6GRRqxHKHfpQJsN7qHKFfBAoO97rCEIFDluH4sgcy6MHcD09xWEErO/SrgfdgZc8C0n4nmOr8GCrdLuaMoMnXlmqI3oG9Yd2eLknLiKW+TI4j3RZoKPNZn7pgYUFXj1jMSfMkP+nZhEwsr49lyW8ABGErPAE8KRmEQDGl1XrghuGOAHiOKnf6eaDLpKFG5Rs0Qt9vGxM+1pV+oTVUg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.059653, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Dec 12, 2024 at 9:51=E2=80=AFPM David Hildenbrand wrote: > > On 12.12.24 09:46, Barry Song wrote: > > On Thu, Dec 12, 2024 at 9:29=E2=80=AFPM Christoph Hellwig wrote: > >> > >> On Thu, Dec 12, 2024 at 08:37:11PM +1300, Barry Song wrote: > >>> From: Barry Song > >>> > >>> While developing the zeromap series, Usama observed that certain > >>> workloads may contain over 10% zero-filled pages. This may present > >>> an opportunity to save memory by mapping zero-filled pages to zero_pf= n > >>> in do_swap_page(). If a write occurs later, do_wp_page() can > >>> allocate a new page using the Copy-on-Write mechanism. > >> > >> Shouldn't this be done during, or rather instead of swap out instead? > >> Swapping all zero pages out just to optimize the in-memory > >> representation on seems rather backwards. > > > > I=E2=80=99m having trouble understanding your point=E2=80=94it seems li= ke you might > > not have fully read the code. :-) > > > > The situation is as follows: for a zero-filled page, we are currently > > allocating a new > > page unconditionally. By mapping this zero-filled page to zero_pfn, we = could > > save the memory used by this page. > > > > We don't need to allocate the memory until the page is written(which ma= y never > > happen). > > I think what Christoph means is that you would determine that at PTE > unmap time, and directly place the zero page in there. So there would be > no need to have the page fault at all. > > I suspect at PTE unmap time might be problematic, because we might still > have other (i.e., GUP) references modifying that page, and we can only > rely on the page content being stable after we flushed the TLB as well. > (I recall some deferred flushing optimizations) Yes, we need to follow a strict sequence: 1. try_to_unmap - unmap PTEs in all processes; 2. try_to_unmap_flush_dirty - flush deferred TLB shootdown; 3. pageout - zeromap will set 1 in bitmap if page is zero-filled At the moment of pageout(), we can be confident that the page is zero-fille= d. mapping to zeropage during unmap seems quite risky. > > -- > Cheers, > > David / dhildenb > Thanks Barry