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@xxxxxxxxxx thanks, -- John Hubbard
