Re: [RFC PATCH] block, fs: use FOLL_LONGTERM as gup_flags for direct IO

[Zi Yan]

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