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

Boaz Harrosh
2019-04-16T18:35:21Z

On Thu, Apr 11, 2019 at 05:08:19PM -0400, jglisse@redhat.com wrote:
> From: Jérôme Glisse <jglisse@redhat.com>
>
> 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.

Cheers
Boaz

> - Third part replace put_page(bv_page(bio_vec)) with a new helper
> which will use put_user_page() when the page in the bio_vec is
> coming from GUP.
> - Fourth part update BIO to use bv_set_user_page() for page that
> are coming from GUP this means updating bio_add_page*() to pass
> down the origin of the page (GUP or not).
> - Fith part convert few more places that directly use bvec_io or
> BIO.
>