On Tue, Jan 14, 2025 at 10:50:49AM -0800, Dan Williams wrote: > Alistair Popple wrote: > > The devmap PTE special bit was used to detect mappings of FS DAX > > pages. This tracking was required to ensure the generic mm did not > > manipulate the page reference counts as FS DAX implemented it's own > > reference counting scheme. > > > > Now that FS DAX pages have their references counted the same way as > > normal pages this tracking is no longer needed and can be > > removed. > > > > Almost all existing uses of pmd_devmap() are paired with a check of > > pmd_trans_huge(). As pmd_trans_huge() now returns true for FS DAX pages > > dropping the check in these cases doesn't change anything. > > > > However care needs to be taken because pmd_trans_huge() also checks that > > a page is not an FS DAX page. This is dealt with either by checking > > !vma_is_dax() or relying on the fact that the page pointer was obtained > > from a page list. This is possible because zone device pages cannot > > appear in any page list due to sharing page->lru with page->pgmap. > > While the patch looks straightforward I think part of taking "care" in > this case is to split it such that any of those careful conversions have > their own bisect point in the history. > > Perhaps this can move to follow-on series to not blow up the patch count > of the base series? ...but first want to get your reaction to splitting > for bisect purposes. TBH I don't feel too strongly about it - I suppose it would make it easier to bisect to the specific case we weren't careful enough about. However I think if a bug is bisected to this particular patch it would be relatively easy based on the context of the bug to narrow it down to a particular file or two. I do however feel strongly about whether or not that should be done in a follow-on series :-) Rebasing such a large series has already become painful and error prone enough so if we want to split this change up it will definitely need to be a separate series done once the rest of this has been merged. So I could be pursaded to roll this and the pfn_t removal (as that depends on devmap going away) together. Let me know what you think.
