Re: [PATCH v4 4/6] io_uring: rsrc: avoid use of vmas parameter in pin_user_pages()

[Jens Axboe <axboe@kernel.dk>]

On 4/19/23 11:23?AM, Lorenzo Stoakes wrote:
> On Wed, Apr 19, 2023 at 10:59:27AM -0600, Jens Axboe wrote:
>> On 4/19/23 10:35?AM, Jens Axboe wrote:
>>> On 4/18/23 9:49?AM, Lorenzo Stoakes wrote:
>>>> We are shortly to remove pin_user_pages(), and instead perform the required
>>>> VMA checks ourselves. In most cases there will be a single VMA so this
>>>> should caues no undue impact on an already slow path.
>>>>
>>>> Doing this eliminates the one instance of vmas being used by
>>>> pin_user_pages().
>>>
>>> First up, please don't just send single patches from a series. It's
>>> really annoying when you are trying to get the full picture. Just CC the
>>> whole series, so reviews don't have to look it up separately.
>>>
>>> So when you're doing a respin for what I'll mention below and the issue
>>> that David found, please don't just show us patch 4+5 of the series.
>>
>> I'll reply here too rather than keep some of this conversaion
>> out-of-band.
>>
>> I don't necessarily think that making io buffer registration dumber and
>> less efficient by needing a separate vma lookup after the fact is a huge
>> deal, as I would imagine most workloads register buffers at setup time
>> and then don't change them. But if people do switch sets at runtime,
>> it's not necessarily a slow path. That said, I suspect the other bits
>> that we do in here, like the GUP, is going to dominate the overhead
>> anyway.
> 
> Thanks, and indeed I expect the GUP will dominate.

Unless you have a lot of vmas... Point is, it's _probably_ not a
problem, but it might and it's making things worse for no real gain as
far as I can tell outside of some notion of "cleaning up the code".

>> My main question is, why don't we just have a __pin_user_pages or
>> something helper that still takes the vmas argument, and drop it from
>> pin_user_pages() only? That'd still allow the cleanup of the other users
>> that don't care about the vma at all, while retaining the bundled
>> functionality for the case/cases that do? That would avoid needing
>> explicit vma iteration in io_uring.
>>
> 
> The desire here is to completely eliminate vmas as an externally available
> parameter from GUP. While we do have a newly introduced helper that returns
> a VMA, doing the lookup manually for all other vma cases (which look up a
> single page and vma), that is more so a helper that sits outside of GUP.
> 
> Having a separate function that still bundled the vmas would essentially
> undermine the purpose of the series altogether which is not just to clean
> up some NULL's but rather to eliminate vmas as part of the GUP interface
> altogether.
> 
> The reason for this is that by doing so we simplify the GUP interface,
> eliminate a whole class of possible future bugs with people holding onto
> pointers to vmas which may dangle and lead the way to future changes in GUP
> which might be more impactful, such as trying to find means to use the fast
> paths in more areas with an eye to gradual eradication of the use of
> mmap_lock.
> 
> While we return VMAs, none of this is possible and it also makes the
> interface more confusing - without vmas GUP takes flags which define its
> behaviour and in most cases returns page objects. The odd rules about what
> can and cannot return vmas under what circumstances are not helpful for new
> users.
> 
> Another point here is that Jason suggested adding a new
> FOLL_ALLOW_BROKEN_FILE_MAPPINGS flag which would, by default, not be
> set. This could assert that only shmem/hugetlb file mappings are permitted
> which would eliminate the need for you to perform this check at all.
> 
> This leads into the larger point that GUP-writing file mappings is
> fundamentally broken due to e.g. GUP not honouring write notify so this
> check should at least in theory not be necessary.
> 
> So it may be the case that should such a flag be added this code will
> simply be deleted at a future point :)

Why don't we do that first then? There's nothing more permanent than a
temporary workaround/fix. Once it's in there, motivation to get rid of
it for most people is zero because they just never see it. Seems like
that'd be a much saner approach rather than the other way around, and
make this patchset simpler/cleaner too as it'd only be removing code in
all of the callers.

-- 
Jens Axboe