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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Oct 18, 2024 at 12:57:08PM GMT, Joanne Koong wrote:
> On Thu, Oct 17, 2024 at 10:57 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > On Thu, Oct 17, 2024 at 06:30:08PM GMT, Joanne Koong wrote:
> > > On Tue, Oct 15, 2024 at 3:01 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
> > > >
> > > > On Mon, 14 Oct 2024 at 20:23, Joanne Koong <joannelkoong@gmail.com> wrote:
> > > >
> > > > > This change sets AS_NO_WRITEBACK_RECLAIM on the inode mapping so that
> > > > > FUSE folios are not reclaimed and waited on while in writeback, and
> > > > > removes the temporary folio + extra copying and the internal rb tree.
> > > >
> > > > What about sync(2)?   And page migration?
> > > >
> > > > Hopefully there are no other cases, but I think a careful review of
> > > > places where generic code waits for writeback is needed before we can
> > > > say for sure.
> > >
> > > The places where I see this potential deadlock still being possible are:
> > > * page migration when handling a page fault:
> > >      In particular, this path: handle_mm_fault() ->
> > > __handle_mm_fault() -> handle_pte_fault() -> do_numa_page() ->
> > > migrate_misplaced_folio() -> migrate_pages() -> migrate_pages_sync()
> > > -> migrate_pages_batch() -> migrate_folio_unmap() ->
> > > folio_wait_writeback()
> >
> > So, this is numa fault and if fuse server is not mapping the fuse folios
> > which it is serving, in its address space then this is not an issue.
> > However hugepage allocation on page fault can cause compaction which
> > might migrate unrelated fuse folios. So, fuse server doing compaction
> > is an issue and we need to resolve similar to reclaim codepath. (Though
> > I think for THP it is not doing MIGRATE_SYNC but doing for gigantic
> > hugetlb pages).
> 
> Thanks for the explanation. Would you mind pointing me to the
> compaction function where this triggers the migrate? Is this in
> compact_zone() where it calls migrate_pages() on the cc->migratepages
> list?
> 

Something like the following:

__alloc_pages_direct_compact() ->
try_to_compact_pages() ->
compact_zone_order() -> /* MIGRATE_ASYNC or MIGRATE_SYNC_LIGHT */
compact_zone() ->
migrate_pages() ->
migrate_pages_sync() ->
migrate_pages_batch() ->
migrate_folio_unmap() ->
folio_wait_writeback()


The following is one code path from hugetlb:

alloc_contig_range_noprof() -> /* MIGRATE_SYNC */
__alloc_contig_migrate_range() ->
migrate_pages() ->
migrate_pages_sync() ->
migrate_pages_batch() ->
migrate_folio_unmap() ->
folio_wait_writeback()