Re: [PATCH v1 00/15] Keep track of GUPed pages in fs and block

[Jerome Glisse]

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