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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, 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 3/13/25 3:49 PM, David Hildenbrand wrote: > On 12.03.25 16:21, Christoph Hellwig wrote: >> On Fri, Mar 07, 2025 at 08:23:08PM +0000, Matthew Wilcox wrote: >>> Howver, the problem is real. >> >> What is the problem? > > I think the problem is the CMA allocation failure, not the latency. > > "if a large amount of direct IO is requested constantly, this can make > pages in CMA pageblocks pinned and unable to migrate outside of the > pageblock" > > We'd need a more reliable way to make CMA allocation -> page migration > make progress. For example, after we isolated the pageblocks and > migration starts doing its thing, we could disallow any further GUP > pins. (e.g., make GUP spin or wait for migration to end) > > We could detect in GUP code that a folio is soon expected to be migrated > by checking the pageblock (isolated) and/or whether the folio is locked. > Jason Gunthorpe and Matthew both had some ideas about how to fix this [1], which were very close (maybe the same) to what you're saying here: sleep and spin in an killable loop. It turns out to be a little difficult to do this--I had trouble making the folio's "has waiters" bit work for this, for example. And then...squirrel! However, I still believe, so far, this is the right approach. I'm just not sure which thing to wait on, exactly. [1] https://lore.kernel.org/20240502183408.GC3341011@nvidia.com thanks, -- John Hubbard