On 15 Mar 2025, at 19:00, David Hildenbrand wrote: > On 15.03.25 02:04, John Hubbard wrote: >> 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. > > Zi Yan has a series to convert the "isolate" state of pageblocks to a separate pageblock bit; it could be considered a lock-bit. Currently, it's essentially the migratetype being MIGRATE_ISOLATE. > > As soon as a pageblock is isolated, one must be prepared for contained pages/folios to get migrated. The folio lock will only be grabbed once actually trying to migrate a folio IIRC, so it might not be the best choice: especially considering allocations that span many pageblocks. > > So maybe one would need a "has waiters" bit per pageblock, so relevant users (e.g., GUP) could wait on the isolate bit getting cleared. The patchset is at: https://lore.kernel.org/linux-mm/20250214154215.717537-1-ziy@xxxxxxxxxx/. I should be able to work on it soon, as I have been busy with folio_split() patchset recently. My patchset extends migratetype bits from 4 to 8 and use bit 7 for MIGRATE_ISOLATE. -- Best Regards, Yan, Zi
