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Date: Wed, 20 Oct 2021 18:06:51 +0100 From: Joao Martins <joao.m.martins@...cle.com> To: Dan Williams <dan.j.williams@...el.com>, Jason Gunthorpe <jgg@...pe.ca> Cc: Matthew Wilcox <willy@...radead.org>, Alex Sierra <alex.sierra@....com>, Andrew Morton <akpm@...ux-foundation.org>, "Kuehling, Felix" <Felix.Kuehling@....com>, Linux MM <linux-mm@...ck.org>, Ralph Campbell <rcampbell@...dia.com>, linux-ext4 <linux-ext4@...r.kernel.org>, linux-xfs <linux-xfs@...r.kernel.org>, amd-gfx list <amd-gfx@...ts.freedesktop.org>, Maling list - DRI developers <dri-devel@...ts.freedesktop.org>, Christoph Hellwig <hch@....de>, Jérôme Glisse <jglisse@...hat.com>, Alistair Popple <apopple@...dia.com>, Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>, Linux NVDIMM <nvdimm@...ts.linux.dev>, David Hildenbrand <david@...hat.com> Subject: Re: [PATCH v1 2/2] mm: remove extra ZONE_DEVICE struct page refcount On 10/19/21 20:21, Dan Williams wrote: > On Tue, Oct 19, 2021 at 9:02 AM Jason Gunthorpe <jgg@...pe.ca> wrote: >> >> On Tue, Oct 19, 2021 at 04:13:34PM +0100, Joao Martins wrote: >>> On 10/19/21 00:06, Jason Gunthorpe wrote: >>>> On Mon, Oct 18, 2021 at 12:37:30PM -0700, Dan Williams wrote: >>>> >>>>>> device-dax uses PUD, along with TTM, they are the only places. I'm not >>>>>> sure TTM is a real place though. >>>>> >>>>> I was setting device-dax aside because it can use Joao's changes to >>>>> get compound-page support. >>>> >>>> Ideally, but that ideas in that patch series have been floating around >>>> for a long time now.. >>>> >>> The current status of the series misses a Rb on patches 6,7,10,12-14. >>> Well, patch 8 too should now drop its tag, considering the latest >>> discussion. >>> >>> If it helps moving things forward I could split my series further into: >>> >>> 1) the compound page introduction (patches 1-7) of my aforementioned series >>> 2) vmemmap deduplication for memory gains (patches 9-14) >>> 3) gup improvements (patch 8 and gup-slow improvements) >> >> I would split it, yes.. >> >> I think we can see a general consensus that making compound_head/etc >> work consistently with how THP uses it will provide value and >> opportunity for optimization going forward. >> I'll go do that. Meanwhile, I'll wait a couple days for Dan to review the dax subsystem patches (6 & 7), or otherwise send them over. >>> Whats the benefit between preventing longterm at start >>> versus only after mounting the filesystem? Or is the intended future purpose >>> to pass more context into an holder potential future callback e.g. nack longterm >>> pins on a page basis? >> >> I understood Dan's remark that the device-dax path allows >> FOLL_LONGTERM and the FSDAX path does not ? >> >> Which, IIRC, today is signaled basd on vma properties and in all cases >> fast-gup is denied. > > Yeah, I forgot that 7af75561e171 eliminated any possibility of > longterm-gup-fast for device-dax, let's not disturb that status quo. > I am slightly confused by this comment -- the status quo is what we are questioning here -- And we talked about changing that in the past too (thread below), that longterm-gup-fast was an oversight that that commit was only applicable to fsdax. [Maybe this is just my english confusion] >>> Maybe we can start by at least not add any flags and just prevent >>> FOLL_LONGTERM on fsdax -- which I guess was the original purpose of >>> commit 7af75561e171 ("mm/gup: add FOLL_LONGTERM capability to GUP fast"). >>> This patch (which I can formally send) has a sketch of that (below scissors mark): >>> >>> https://lore.kernel.org/linux-mm/6a18179e-65f7-367d-89a9-d5162f10fef0@oracle.com/ >> >> Yes, basically, whatever test we want for 'deny fast gup foll >> longterm' is fine. >> >> Personally I'd like to see us move toward a set of flag specifying >> each special behavior and not a collection of types that imply special >> behaviors. >> >> Eg we have at least: >> - Block gup fast on foll_longterm >> - Capture the refcount ==1 and use the pgmap free hook >> (confusingly called page_is_devmap_managed()) >> - Always use a swap entry >> - page->index/mapping are used in the usual file based way? >> >> Probably more things.. > > Yes, agree with the principle of reducing type-implied special casing. > OK. Moving from implicit devmap types to pgmap::flags is rather simple fixup. And I suppose (respectivally) PGMAP_NO_PINF_LONGTERM, PGMAP_MANAGED_FREE_PAGE, PGMAP_USE_SWAP_ENTRY, etc, etc.
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