On Tue, Feb 23, 2021 at 10:54 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > On Tue, Feb 23, 2021 at 08:44:52AM -0800, Dan Williams wrote: > > > > The downside would be one extra lookup in dev_pagemap tree > > > for other pgmap->types (P2P, FSDAX, PRIVATE). But just one > > > per gup-fast() call. > > > > I'd guess a dev_pagemap lookup is faster than a get_user_pages slow > > path. It should be measurable that this change is at least as fast or > > faster than falling back to the slow path, but it would be good to > > measure. > > What is the dev_pagemap thing doing in gup fast anyhow? > > I've been wondering for a while.. It's there to synchronize against dax-device removal. The device will suspend removal awaiting all page references to be dropped, but gup-fast could be racing device removal. So gup-fast checks for pte_devmap() to grab a live reference to the device before assuming it can pin a page.
