Re: [PATCH v2 2/2] fuse: remove tmp folio for writebacks and internal rb tree

[Joanne Koong <joannelkoong@gmail.com>]

On Fri, Nov 1, 2024 at 4:44 AM Jingbo Xu <jefflexu@linux.alibaba.com> wrote:
>
> Hi Joanne,
>
> Thanks for keeping pushing this forward.
>
> On 11/1/24 5:52 AM, Joanne Koong wrote:
> > On Thu, Oct 31, 2024 at 1:06 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >>
> >> On Thu, Oct 31, 2024 at 12:06:49PM GMT, Joanne Koong wrote:
> >>> On Wed, Oct 30, 2024 at 5:30 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >> [...]
> >>>>
> >>>> Memory pool is a bit confusing term here. Most probably you are asking
> >>>> about the migrate type of the page block from which tmp page is
> >>>> allocated from. In a normal system, tmp page would be allocated from page
> >>>> block with MIGRATE_UNMOVABLE migrate type while the page cache page, it
> >>>> depends on what gfp flag was used for its allocation. What does fuse fs
> >>>> use? GFP_HIGHUSER_MOVABLE or something else? Under low memory situation
> >>>> allocations can get mixed up with different migrate types.
> >>>>
> >>>
> >>> I believe it's GFP_HIGHUSER_MOVABLE for the page cache pages since
> >>> fuse doesn't set any additional gfp masks on the inode mapping.
> >>>
> >>> Could we just allocate the fuse writeback pages with GFP_HIGHUSER
> >>> instead of GFP_HIGHUSER_MOVABLE? That would be in fuse_write_begin()
> >>> where we pass in the gfp mask to __filemap_get_folio(). I think this
> >>> would give us the same behavior memory-wise as what the tmp pages
> >>> currently do,
> >>
> >> I don't think it would be the same behavior. From what I understand the
> >> liftime of the tmp page is from the start of the writeback till the ack
> >> from the fuse server that writeback is done. While the lifetime of the
> >> page of the page cache can be arbitrarily large. We should just make it
> >> unmovable for its lifetime. I think it is fine to make the page
> >> unmovable during the writeback. We should not try to optimize for the
> >> bad or buggy behavior of fuse server.
> >>
> >> Regarding the avoidance of wait on writeback for fuse folios, I think we
> >> can handle the migration similar to how you are handling reclaim and in
> >> addition we can add a WARN() in folio_wait_writeback() if the kernel ever
> >> sees a fuse folio in that function.
> >
> > Awesome, this is what I'm planning to do in v3 to address migration then:
> >
> > 1) in migrate_folio_unmap(), only call "folio_wait_writeback(src);" if
> > src->mapping does not have the AS_NO_WRITEBACK_WAIT bit set on it (eg
> > fuse folios will have that AS_NO_WRITEBACK_WAIT bit set)
>
> I think it's generally okay to skip FUSE pages under writeback when the
> sync migrate_pages() is called in low memory context, which only tries
> to migrate as many pages as possible (i.e. best effort).
>
> While more caution may be needed when the sync migrate_pages() is called
> with an implicit hint that the migration can not fail.  For example,
>
> ```
> offline_pages
>         while {
>                 scan_movable_pages
>                 do_migrate_range
>         }
> ```
>
> If the malicious server never completes the writeback IO, no progress
> will be made in the above while loop, and I'm afraid it will be a dead
> loop then.
>

Thanks for taking a look and sharing your thoughts.
I agree. I think for this offline_pages() path, we need to handle this
"TODO: fatal migration failures should bail out". For v3 I'm thinking
of handling this by having some number of retries where we try
do_migrate_range() but if it still doesn't succeed, to skip those
pages and move onto the next.

>
> >
> > 2) in the fuse filesystem's implementation of the
> > mapping->a_ops->migrate_folio callback, return -EAGAIN if the folio is
> > under writeback.
>
> Is there any possibility that a_ops->migrate_folio() may be called with
> the folio under writeback?
>
> - for most pages without AS_NO_WRITEBACK_WAIT, a_ops->migrate_folio()
> will be called only when Page_writeback is cleared;
> - for AS_NO_WRITEBACK_WAIT pages, they are skipped if they are under
> writeback
>

For AS_NO_WRITEBACK_WAIT_PAGES, if we skip waiting on them if they are
under writeback, I think the a_ops->migrate_folio() will still get
called (by migrate_pages_batch() -> migrate_folio_move() ->
move_to_new_folio()).

Looking at migrate_folio_unmap() some more though,  I don't think we
can just skip the wait call like we can for the sync(2) case. I think
we need to error out here instead since after the wait call,
migrate_folio_unmap() will replace the folio's page table mappings
(try_to_migrate()). If we error out here, then there's no hitting
a_ops->migrate_folio() when the folio is under writeback.


Thanks,
Joanne
> --
> Thanks,
> Jingbo