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

[Dan Williams]

On Fri, Dec 7, 2018 at 11:16 AM Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
>
> On Thu, Dec 06, 2018 at 06:45:49PM -0800, John Hubbard wrote:
> > On 12/4/18 5:57 PM, John Hubbard wrote:
> > > On 12/4/18 5:44 PM, Jerome Glisse wrote:
> > >> On Tue, Dec 04, 2018 at 05:15:19PM -0800, Matthew Wilcox wrote:
> > >>> On Tue, Dec 04, 2018 at 04:58:01PM -0800, John Hubbard wrote:
> > >>>> On 12/4/18 3:03 PM, Dan Williams wrote:
> > >>>>> Except the LRU fields are already in use for ZONE_DEVICE pages... how
> > >>>>> does this proposal interact with those?
> > >>>>
> > >>>> Very badly: page->pgmap and page->hmm_data both get corrupted. Is there an entire
> > >>>> use case I'm missing: calling get_user_pages() on ZONE_DEVICE pages? Said another
> > >>>> way: is it reasonable to disallow calling get_user_pages() on ZONE_DEVICE pages?
> > >>>>
> > >>>> If we have to support get_user_pages() on ZONE_DEVICE pages, then the whole
> > >>>> LRU field approach is unusable.
> > >>>
> > >>> We just need to rearrange ZONE_DEVICE pages.  Please excuse the whitespace
> > >>> damage:
> > >>>
> > >>> +++ b/include/linux/mm_types.h
> > >>> @@ -151,10 +151,12 @@ struct page {
> > >>>  #endif
> > >>>                 };
> > >>>                 struct {        /* ZONE_DEVICE pages */
> > >>> +                       unsigned long _zd_pad_2;        /* LRU */
> > >>> +                       unsigned long _zd_pad_3;        /* LRU */
> > >>> +                       unsigned long _zd_pad_1;        /* uses mapping */
> > >>>                         /** @pgmap: Points to the hosting device page map. */
> > >>>                         struct dev_pagemap *pgmap;
> > >>>                         unsigned long hmm_data;
> > >>> -                       unsigned long _zd_pad_1;        /* uses mapping */
> > >>>                 };
> > >>>
> > >>>                 /** @rcu_head: You can use this to free a page by RCU. */
> > >>>
> > >>> You don't use page->private or page->index, do you Dan?
> > >>
> > >> page->private and page->index are use by HMM DEVICE page.
> > >>
> > >
> > > OK, so for the ZONE_DEVICE + HMM case, that leaves just one field remaining for
> > > dma-pinned information. Which might work. To recap, we need:
> > >
> > > -- 1 bit for PageDmaPinned
> > > -- 1 bit, if using LRU field(s), for PageDmaPinnedWasLru.
> > > -- N bits for a reference count
> > >
> > > Those *could* be packed into a single 64-bit field, if really necessary.
> > >
> >
> > ...actually, this needs to work on 32-bit systems, as well. And HMM is using a lot.
> > However, it is still possible for this to work.
> >
> > Matthew, can I have that bit now please? I'm about out of options, and now it will actually
> > solve the problem here.
> >
> > Given:
> >
> > 1) It's cheap to know if a page is ZONE_DEVICE, and ZONE_DEVICE means not on the LRU.
> > That, in turn, means only 1 bit instead of 2 bits (in addition to a counter) is required,
> > for that case.
> >
> > 2) There is an independent bit available (according to Matthew).
> >
> > 3) HMM uses 4 of the 5 struct page fields, so only one field is available for a counter
> >    in that case.
>
> To expend on this, HMM private page are use for anonymous page
> so the index and mapping fields have the value you expect for
> such pages. Down the road i want also to support file backed
> page with HMM private (mapping, private, index).
>
> For HMM public both anonymous and file back page are supported
> today (HMM public is only useful on platform with something like
> OpenCAPI, CCIX or NVlink ... so PowerPC for now).
>
> > 4) get_user_pages() must work on ZONE_DEVICE and HMM pages.
>
> get_user_pages() only need to work with HMM public page not the
> private one as we can not allow _anyone_ to pin HMM private page.

How does HMM enforce that? Because the kernel should not allow *any*
memory management facility to arbitrarily fail direct-I/O operations.
That's why CONFIG_FS_DAX_LIMITED is a temporary / experimental hack
for S390 and ZONE_DEVICE was invented to bypass that hack for X86 and
any arch that plans to properly support DAX. I would classify any
memory management that can't support direct-I/O in the same
"experimental" category.