Re: [PATCH v6 23/26] mm: Remove pXX_devmap callers

Alistair Popple
2025-02-05T09:59:27Z

On Tue, Feb 04, 2025 at 11:06:08AM -0800, Dan Williams wrote:
> Alistair Popple wrote:
> > 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.
>
> I tend to think that there's never any regrets for splitting a patch
> along lines of risk. I am fine with keeping that in this series if that
> makes things easier.

Yes, that is a reaonable point of view. You will notice I dropped these
clean-ups in my latest repost as I intend to post them as a separate clean-up
series to be applied on top of this one. My hope would be the clean up series
would also make it into v6.15.