On Tue 21-12-21 18:40:30, David Hildenbrand wrote: > On 21.12.21 18:05, Linus Torvalds wrote: > > On Tue, Dec 21, 2021 at 12:58 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > >> Note that I am trying to make also any kind of R/O pins on an anonymous > >> page work as expected as well, to fix any kind of GUP after fork() and > >> GUP before fork(). So taking a R/O pin on an !PageAnonExclusive() page > >> similarly has to make sure that the page is exclusive -- even if it's > >> mapped R/O (!). > > > > I do think the existing "maybe_pinned()" logic is fine for that. The > > "exclusive to this VM" bit can be used to *help* that decision - > > because only an exclusive page can be pinned - bit I don't think it > > should _replace_ that logic. > > The issue is that O_DIRECT uses FOLL_GET and cannot easily be changed to > FOLL_PIN unfortunately. So I'm *trying* to make it more generic such > that such corner cases can be handled as well correctly. But yeah, I'll > see where this goes ... O_DIRECT has to be fixed one way or the other. > > John H. mentioned that he wants to look into converting that to > FOLL_PIN. So maybe that will work eventually. For record we always intended (and still intend) to make O_DIRECT use FOLL_PIN. Just it is tricky because some users mix pages pinned with GUP and pages acquired through get_page() in a single bio (such as zero page) and thus it is non-trivial to do the right thing on IO completion (unpin or just put_page). Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
