On Tue, Apr 16, 2019 at 09:35:04PM +0300, Boaz Harrosh wrote: > On Thu, Apr 11, 2019 at 05:08:19PM -0400, jglisse@xxxxxxxxxx wrote: > > From: Jérôme Glisse <jglisse@xxxxxxxxxx> > > > > This patchset depends on various small fixes [1] and also on patchset > > which introduce put_user_page*() [2] and thus is 5.3 material as those > > pre-requisite will get in 5.2 at best. Nonetheless i am posting it now > > so that it can get review and comments on how and what should be done > > to test things. > > > > For various reasons [2] [3] we want to track page reference through GUP > > differently than "regular" page reference. Thus we need to keep track > > of how we got a page within the block and fs layer. To do so this patch- > > set change the bio_bvec struct to store a pfn and flags instead of a > > direct pointer to a page. This way we can flag page that are coming from > > GUP. > > > > This patchset is divided as follow: > > - First part of the patchset is just small cleanup i believe they > > can go in as his assuming people are ok with them. > > > > - Second part convert bio_vec->bv_page to bio_vec->bv_pfn this is > > done in multi-step, first we replace all direct dereference of > > the field by call to inline helper, then we introduce macro for > > bio_bvec that are initialized on the stack. Finaly we change the > > bv_page field to bv_pfn. > > Why do we need a bv_pfn. Why not just use the lowest bit of the page-ptr > as a flag (pointer always aligned to 64 bytes in our case). > > So yes we need an inline helper for reference of the page but is it not clearer > that we assume a page* and not any kind of pfn ? > It will not be the first place using low bits of a pointer for flags. > > That said. Why we need it at all? I mean why not have it as a bio flag. If it exist > at all that a user has a GUP and none-GUP pages to IO at the same request he/she > can just submit them as two separate BIOs (chained at the block layer). > > Many users just submit one page bios and let elevator merge them any way. Let's please not add additional flags and weirdness to struct bio - "if this flag is set interpret one way, if not interpret another" - or eventually bios will be as bad as skbuffs. I would much prefer just changing bv_page to bv_pfn. Question though - why do we need a flag for whether a page is a GUP page or not? Couldn't the needed information just be determined by what range the pfn is not (i.e. whether or not it has a struct page associated with it)?
