Messages in this thread | | | Subject | Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork() | From | John Hubbard <> | Date | Tue, 22 Sep 2020 13:01:13 -0700 |
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On 9/22/20 3:33 AM, Jan Kara wrote: > On Mon 21-09-20 23:41:16, John Hubbard wrote: >> On 9/21/20 2:20 PM, Peter Xu wrote: >> ... >>> + if (unlikely(READ_ONCE(src_mm->has_pinned) && >>> + page_maybe_dma_pinned(src_page))) { >> >> This condition would make a good static inline function. It's used in 3 >> places, and the condition is quite special and worth documenting, and >> having a separate function helps with that, because the function name >> adds to the story. I'd suggest approximately: >> >> page_likely_dma_pinned() >> >> for the name. > > Well, but we should also capture that this really only works for anonymous > pages. For file pages mm->has_pinned does not work because the page may be > still pinned by completely unrelated process as Jann already properly > pointed out earlier in the thread. So maybe anon_page_likely_pinned()? > Possibly also assert PageAnon(page) in it if we want to be paranoid... > > Honza
The file-backed case doesn't really change anything, though: page_maybe_dma_pinned() is already a "fuzzy yes" in the same sense: you can get a false positive. Just like here, with an mm->has_pinned that could be a false positive for a process.
And for that reason, I'm also not sure an "assert PageAnon(page)" is desirable. That assertion would prevent file-backed callers from being able to call a function that provides a fuzzy answer, but I don't see why you'd want or need to do that. The goal here is to make the fuzzy answer a little bit more definite, but it's not "broken" just because the result is still fuzzy, right?
Apologies if I'm missing a huge point here... :)
thanks, -- John Hubbard NVIDIA
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