On Wed, Mar 20, 2024 at 07:24:27AM +0000, Yosry Ahmed wrote: > [..] > > > > - /* map */ > > > > - spin_lock(&tree->lock); > > > > /* > > > > - * The folio may have been dirtied again, invalidate the > > > > - * possibly stale entry before inserting the new entry. > > > > + * We finish initializing the entry while it's already in xarray. > > > > + * This is safe because: > > > > + * > > > > + * 1. Concurrent stores and invalidations are excluded by folio lock. > > > > + * > > > > + * 2. Writeback is excluded by the entry not being on the LRU yet. > > > > + * The publishing order matters to prevent writeback from seeing > > > > + * an incoherent entry. > > > > > > As I mentioned before, writeback is also protected by the folio lock. > > > Concurrent writeback will find the folio in the swapcache and abort. The > > > fact that the entry is not on the LRU yet is just additional protection, > > > so I don't think the publishing order actually matters here. Right? > > > > Right. This comment is explaining why this publishing order does not > > matter. I think we are talking about the same thing here? > > The comment literally says "the publishing order matters.." :) > > I believe Johannes meant that we should only publish the entry to the > LRU once it is fully initialized, to prevent writeback from using a > partially initialized entry. > > What I am saying is that, even if we add a partially initialized entry > to the zswap LRU, writeback will skip it anyway because the folio is > locked in the swapcache. > > So basically I think the comment should say: > > /* > * We finish initializing the entry while it's already in the > * xarray. This is safe because the folio is locked in the swap > * cache, which should protect against concurrent stores, > * invalidations, and writeback. > */ > > Johannes, what do you think? I don't think that's quite right. Writeback will bail on swapcache insert, yes, but it will access the entry before attempting it. If LRU publishing happened before setting entry->swpentry e.g., we'd have a problem, while your comment suggets it would be safe to rearrange the code like this. So LRU publishing order does matter.
