Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

From: John Hubbard
Date: Wed Dec 12 2018 - 14:08:01 EST

On 12/12/18 9:49 AM, Dan Williams wrote:
> On Wed, Dec 12, 2018 at 9:02 AM Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
>> On Wed, Dec 12, 2018 at 08:27:35AM -0800, Dan Williams wrote:
>>> On Wed, Dec 12, 2018 at 7:03 AM Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
>>>> On Mon, Dec 10, 2018 at 11:28:46AM +0100, Jan Kara wrote:
>>>>> On Fri 07-12-18 21:24:46, Jerome Glisse wrote:
>>>>>> Another crazy idea, why not treating GUP as another mapping of the page
>>>>>> and caller of GUP would have to provide either a fake anon_vma struct or
>>>>>> a fake vma struct (or both for PRIVATE mapping of a file where you can
>>>>>> have a mix of both private and file page thus only if it is a read only
>>>>>> GUP) that would get added to the list of existing mapping.
>>>>>> So the flow would be:
>>>>>> somefunction_thatuse_gup()
>>>>>> {
>>>>>> ...
>>>>>> GUP(_fast)(vma, ..., fake_anon, fake_vma);
>>>>>> ...
>>>>>> }
>>>>>> GUP(vma, ..., fake_anon, fake_vma)
>>>>>> {
>>>>>> if (vma->flags == ANON) {
>>>>>> // Add the fake anon vma to the anon vma chain as a child
>>>>>> // of current vma
>>>>>> } else {
>>>>>> // Add the fake vma to the mapping tree
>>>>>> }
>>>>>> // The existing GUP except that now it inc mapcount and not
>>>>>> // refcount
>>>>>> GUP_old(..., &nanonymous, &nfiles);
>>>>>> atomic_add(&fake_anon->refcount, nanonymous);
>>>>>> atomic_add(&fake_vma->refcount, nfiles);
>>>>>> return nanonymous + nfiles;
>>>>>> }
>>>>> Thanks for your idea! This is actually something like I was suggesting back
>>>>> at LSF/MM in Deer Valley. There were two downsides to this I remember
>>>>> people pointing out:
>>>>> 1) This cannot really work with __get_user_pages_fast(). You're not allowed
>>>>> to get necessary locks to insert new entry into the VMA tree in that
>>>>> context. So essentially we'd loose get_user_pages_fast() functionality.
>>>>> 2) The overhead e.g. for direct IO may be noticeable. You need to allocate
>>>>> the fake tracking VMA, get VMA interval tree lock, insert into the tree.
>>>>> Then on IO completion you need to queue work to unpin the pages again as you
>>>>> cannot remove the fake VMA directly from interrupt context where the IO is
>>>>> completed.
>>>>> You are right that the cost could be amortized if gup() is called for
>>>>> multiple consecutive pages however for small IOs there's no help...
>>>>> So this approach doesn't look like a win to me over using counter in struct
>>>>> page and I'd rather try looking into squeezing HMM public page usage of
>>>>> struct page so that we can fit that gup counter there as well. I know that
>>>>> it may be easier said than done...
>>>> So i want back to the drawing board and first i would like to ascertain
>>>> that we all agree on what the objectives are:
>>>> [O1] Avoid write back from a page still being written by either a
>>>> device or some direct I/O or any other existing user of GUP.
>>>> This would avoid possible file system corruption.
>>>> [O2] Avoid crash when set_page_dirty() is call on a page that is
>>>> considered clean by core mm (buffer head have been remove and
>>>> with some file system this turns into an ugly mess).
>>>> [O3] DAX and the device block problems, ie with DAX the page map in
>>>> userspace is the same as the block (persistent memory) and no
>>>> filesystem nor block device understand page as block or pinned
>>>> block.
>>>> For [O3] i don't think any pin count would help in anyway. I believe
>>>> that the current long term GUP API that does not allow GUP of DAX is
>>>> the only sane solution for now.
>>> No, that's not a sane solution, it's an emergency hack.
>> Then how do you want to solve it ? Knowing pin count does not help
>> you, at least i do not see how that would help and if it does then
>> my solution allow you to know pin count it is the difference between
>> real mapping and mapcount value.
> True, pin count doesn't help, and indefinite waits are intolerable, so
> I think we need to make "long term" GUP revokable, but otherwise
> hopefully use the put_user_page() scheme to replace the use of the pin
> count for dax_layout_busy_page().
>>>> The real fix would be to teach file-
>>>> system about DAX/pinned block so that a pinned block is not reuse
>>>> by filesystem.
>>> We already have taught filesystems about pinned dax pages, see
>>> dax_layout_busy_page(). As much as possible I want to eliminate the
>>> concept of "dax pages" as a special case that gets sprinkled
>>> throughout the mm.
>>>> For [O1] and [O2] i believe a solution with mapcount would work. So
>>>> no new struct, no fake vma, nothing like that. In GUP for file back
>>>> pages
>>> With get_user_pages_fast() we don't know that we have a file-backed
>>> page, because we don't have a vma.
>> You do not need a vma to know that we have PageAnon() for that so my
>> solution is just about adding to core GUP page table walker:
>> if (!PageAnon(page))
>> atomic_inc(&page->mapcount);
> Ah, ok, would need to add proper mapcount manipulation for dax and
> audit that nothing makes page-cache assumptions based on a non-zero
> mapcount.
>> Then in put_user_page() you add the opposite. In page_mkclean() you
>> count the number of real mapping and voilà ... you got an answer for
>> [O1]. You could use the same count real mapping to get the pin count
>> in other place that cares about it but i fails to see why the actual
>> pin count value would matter to any one.
> Sounds like a could work... devil is in the details.

I also like this solution. It uses existing information and existing pointers
(PageAnon, page_mapping) plus a tiny bit more (another pointer in struct
address_space, probably, and that's not a size-contentious struct) to figure
out the DMA pinned count, which neatly avoids the struct page contention that I
ran into.

Thanks for coming up with this! I'll put together a patchset that shows the details
so we can all take a closer look. This is on the top of my list.

John Hubbard