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

[Jerome Glisse]

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.

Yes i can use the lower bit of struct page * pointer it should be safe on
all architecture. I wanted to change the bv_page field name to make sure
that we catch anyone doing any direct dereference. Do you prefer keeping a
page pointer there ?

> 
> 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.

The issue is that bio_vec is use, on its own, outside of bios and for
those use cases i need to track the GUP status within the bio_vec. Thus
it is easier to use the same mechanisms for bio too as adding a flag to
bio would mean that i also have to audit all code path that could merge
bios. While i believe it should be restrictred to block/blk-merge.c it
seems some block and some fs have spawn some custom bio manipulation
(md comes to mind). So using same mechanism for bio_vec and bio seems
like a safer and easier course of action.

Cheers,
Jérôme