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@xxxxxxxxxxxxxxxxxxxx/ > (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. 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
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