On Wed, Sep 09, 2020 at 09:01:14AM +0100, Christoph Hellwig wrote: > (1) a FOLL_HUGEPAGE flag for the pin_user_pages API family that returns > a single struct page for any kind of huge page, which would also > benefit all kinds of other users rather than adding these kinds of > hacks to vfio. > (2) add a bvec version of the API that returns a variable size > "extent" > > I had started on (2) a while ago, and here is branch with my code (which > is broken and fails test, but might be a start): > > http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/gup-bvec > > But for now I wonder if (1) is the better start, which could still be > reused to (2) later. (1) doesn't work. Suppose you have a 16kB MAP_PRIVATE of a file which happens to have a THP in the page cache. When you write a byte, that base page gets copied and your bvec for that 16kB needs to look something like: { page A, offset 0, length 4096 }, { page B, offset 0, length 4096 }, { page A, offset 8192, length 8192 }, You can come up with other scenarios, like partial mappings of THPs with an adjacent VMA of a different THP, but I think we just need to make (2) work.
