Re: [PATCH v2] mm/migrate: put dest folio on deferred split list if source was there.

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 12, 2024 at 11:51:13AM -0400, Zi Yan wrote:
> On 12 Mar 2024, at 10:19, Matthew Wilcox wrote:
> 
> > On Tue, Mar 12, 2024 at 10:13:16AM -0400, Zi Yan wrote:
> >> On 11 Mar 2024, at 23:45, Matthew Wilcox wrote:
> >>> Much more important: You're doing this with a positive refcount, which
> >>> breaks the (undocumented) logic in deferred_split_scan() that a folio
> >>> with a positive refcount will not be removed from the list.
> >>
> >> What is the issue here? I thought as long as the split_queue_lock is held,
> >> it should be OK to manipulate the list.
> >
> > I just worked this out yesterday:
> > https://lore.kernel.org/linux-mm/Ze9EFdFLXQEUVtKl@casper.infradead.org/
> > (the last chunk, starting with Ryan asking me "what about the first bug
> > you found")
> 
> Hmm, like you said a folio with a positive refcount will not be removed
> from ds_queue->split_queue, it will have no chance going to the separate
> list in deferred_list_scan() and list_del_init() will not corrupt
> that list.

You've misread it.  Folios with a _zero_ refcount are not removed from
the list in deferred_split_scan.  Folios with a positive refcount are
removed from the per-node or per-cgroup list _at which point there is
an undocumented assumption_ that they will not be removed from the
local list because they have a positive refcount.

> So it should be safe. Or the issue is that before migration
> adding a refcount, the folio is removed from ds_queue->split_queue
> and put on the list in deferred_list_scan(), as a result, any manipulation
> of folio->_deferred_list could corrupt the list. Basically,
> !list_empty(&folio->_deferred_list) cannot tell if the folio is on
> ds_queue->split_queue or another list. I am not sure about why "a positive
> refcount" is related here.
> 
> That makes me wonder whether ds_queue->split_queue_lock is also needed
> for list_for_each_entry_safe() in deferred_split_scan(). Basically,
> ds_queue->split_queue_lock protects folio->_deferred_list in addition to
> ds_queue->split_queue.
> 
> 
> 
> >>> Maximally important: Wer shouldn't be doing any of this!  This folio is
> >>> on the deferred split list.  We shouldn't be migrating it as a single
> >>> entity; we should be splitting it now that we're in a context where we
> >>> can do the right thing and split it.  Documentation/mm/transhuge.rst
> >>> is clear that we don't split it straight away due to locking context.
> >>> Splitting it on migration is clearly the right thing to do.
> >>>
> >>> If splitting fails, we should just fail the migration; splitting fails
> >>> due to excess references, and if the source folio has excess references,
> >>> then migration would fail too.
> >>
> >> You are suggesting:
> >> 1. checking if the folio is on deferred split list or not
> >> 2. if yes, split the folio
> >> 3. if split fails, fail the migration as well.
> >>
> >> It sounds reasonable to me. The split folios should be migrated since
> >> the before-split folio wants to be migrated. This split is not because
> >> no new page cannot be allocated, thus the split folios should go
> >> into ret_folios list instead of split_folios list.
> >
> > Yes, I'm happy for the split folios to be migrated.  Bonus points if you
> > want to figure out what order to split the folio to ;-)  I don't think
> > it's critical.
> 
> 
> --
> Best Regards,
> Yan, Zi