On Tue, Apr 16, 2019 at 12:12:27PM -0700, Dan Williams wrote: > On Tue, Apr 16, 2019 at 11:59 AM Kent Overstreet > <kent.overstreet@xxxxxxxxx> wrote: > > > > 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. > > This all reminds of the failed attempt to teach the block layer to > operate without pages: > > https://lore.kernel.org/lkml/20150316201640.33102.33761.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > > > > > 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)? > > That amounts to a pfn_valid() check which is a bit heavier than if we > can store a flag in the bv_pfn entry directly. > > I'd say create a new PFN_* flag, and make bv_pfn a 'pfn_t' rather than > an 'unsigned long'. > > That said, I'm still in favor of Jan's proposal to just make the > bv_page semantics uniform. Otherwise we're complicating this core > infrastructure for some yet to be implemented GPU memory management > capabilities with yet to be determined value. Circle back when that > value is clear, but in the meantime fix the GUP bug. This has nothing to do with GPU, what make you think so ? Here i am trying to solve GUP and to keep the value of knowing wether a page has been GUP or not. I argue that if we bias every page in every bio then we loose that information and thus the value. I gave the page protection mechanisms as an example that would be impacted but it is not the only one. Knowing if a page has been GUP can be useful for memory reclaimation, compaction, NUMA balancing, ... Also page that are going through a bio in one thread might be under some other fs specific operation in another thread which would be block by GUP but do not need to be block by I/O (ie fs can either wait on the I/O or knows that it is safe to proceed even if the page is under I/O). Hence i believe that by making every page look the same we do loose valuable information. More over the complexity of making all the page in bio have a reference count bias is much bigger than the changes needed to keep track of wether the page did came from GUP or not. Cheers, Jérôme
