Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

[David Hildenbrand]

On 22.12.21 15:42, Jan Kara wrote:
> On Wed 22-12-21 14:09:41, David Hildenbrand wrote:
>>>> IIUC, our COW logic makes sure that a shared anonymous page that might
>>>> still be used by a R/O FOLL_GET cannot be modified, because any attempt
>>>> to modify it would result in a copy.
>>>
>>> Well, we defined FOLL_PIN to mean the intent that the caller wants to access
>>> not only page state (for which is enough FOLL_GET and there are some users
>>> - mostly inside mm - who need this) but also page data. Eventually, we even
>>> wanted to make FOLL_GET unavailable to broad areas of kernel (and keep it
>>> internal to only MM for its dirty deeds ;)) to reduce the misuse of GUP.
>>>
>>> For file pages we need this data vs no-data access distinction so that
>>> filesystems can detect when someone can be accessing page data although the
>>> page is unmapped.  Practically, filesystems care most about when someone
>>> can be *modifying* page data (we need to make sure data is stable e.g. when
>>> writing back data to disk or doing data checksumming or other operations)
>>> so using FOLL_GET when wanting to only read page data should be OK for
>>> filesystems but honestly I would be reluctant to break the rule of "use
>>> FOLL_PIN when wanting to access page data" to keep things simple and
>>> reasonably easy to understand for parties such as filesystem developers or
>>> driver developers who all need to interact with pinned pages...
>>
>> Right, from an API perspective we really want people to use FOLL_PIN.
>>
>> To optimize this case in particular it would help if we would have the
>> FOLL flags on the unpin path. Then we could just decide internally
>> "well, short-term R/O FOLL_PIN can be really lightweight, we can treat
>> this like a FOLL_GET instead". And we would need that as well if we were
>> to keep different counters for R/O vs. R/W pinned.
> 
> Well, I guess the question here is: Which GUP user needs only R/O access to
> page data and is so performance critical that it would be worth it to
> sacrifice API clarity for speed? I'm not aware of any but I was not looking
> really hard...

I'd be interested in examples as well. Maybe databases that use O_DIRECT
after fork()?


-- 
Thanks,

David / dhildenb