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dkim=pass header.d=Nvidia.com header.s=selector2 header.b=sTAkL3bn; spf=none (imf12.hostedemail.com: domain of jhubbard@nvidia.com has no SPF policy when checking 40.107.244.61) smtp.mailfrom=jhubbard@nvidia.com; dmarc=pass (policy=reject) header.from=nvidia.com X-HE-Tag: 1643339140-840879 X-Bogosity: Ham, tests=bogofilter, spamicity=0.023733, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 1/27/22 13:57, Matthew Wilcox wrote: > As promised, here's a half-baked proposal for making folio_mapcount() > significantly cheaper at the cost of making it less precise. > I appreciate that folio_mapcount() is not upstream yet, so take a look > at total_mapcount() if you want to understand what I'm talking about. > > For a 2MB folio on a 4k architecture, you have to check 512 cachelines > to determine how many times a folio is mapped. That's 32kB of memory, > which is a good chunk of your L1 cache. The problem is that every PTE > mapping increments the ->mapcount of each individual page (and the number > of PMD mappings is stored separately). To find out how many times the > entire folio is mapped, you've got to look at each constituent page. > > Added to that, each increment of any of the ->mapcount bumps the > refcount on the head page. That's a lot of atomic ops, and we've had > some problems where the page refcount has been attacked resulting in > overflow. > > I would like to start counting folio mapcounts in a more Discworld Troll > manner. Zero, One, Two, Many. That limits the total number of refcount > increments to 3. Once you reach "Many", you've essentially lost count, > and you need to walk the interval tree to figure out exactly how many > mappings there are (this means we can no longer use mapcount to decide to > stop walking the rmap, but I think that's OK?) You can decrement from > Two to One and One to Zero, but you can't decrement from Many to Two. > If you walk the rmap and discover there are less than Many mappings, > you can set mapcount to Two, One or Zero (adjusting page refcount at > the same time). > > The mapcount would also no longer count the number of individual PTE or > PMD mappings. Instead, it would be the number of VMAs which contain at > least one page table reference to this folio. > > One advantage to this scheme is that it makes something like 30 bits > available in struct page. I'm sure we'll be able to think of some good > uses for them. PageDoubleMap also goes away (because we no longer care Such as upgrading from: page_maybe_dma_pinned(), to: oh_yes_page_is_most_definitely_dma_pinned() ! :) ...I just can't let that idea go. haha. thanks, -- John Hubbard NVIDIA > whether the folio is mapped with PMDs or PTEs). > > So ... what's going to be made catastrophically slower by this scheme? > Maybe something involving anonymous pages? Those tend to be my blind > spot. >