Re: [PATCH v3 6/8] filemap: Allow __filemap_get_folio to allocate large folios

Matthew Wilcox
2023-06-13T02:00:22Z

On Tue, Jun 13, 2023 at 11:30:13AM +1000, Dave Chinner wrote:
> On Tue, Jun 13, 2023 at 01:42:51AM +0100, Matthew Wilcox wrote:
> > On Tue, Jun 13, 2023 at 08:49:05AM +1000, Dave Chinner wrote:
> > > On Mon, Jun 12, 2023 at 09:39:08PM +0100, Matthew Wilcox (Oracle) wrote:
> > > > Allow callers of __filemap_get_folio() to specify a preferred folio
> > > > order in the FGP flags. This is only honoured in the FGP_CREATE path;
> > > > if there is already a folio in the page cache that covers the index,
> > > > we will return it, no matter what its order is. No create-around is
> > > > attempted; we will only create folios which start at the specified index.
> > > > Unmodified callers will continue to allocate order 0 folios.
> > > .....
> > > > - /* Init accessed so avoid atomic mark_page_accessed later */
> > > > - if (fgp_flags & FGP_ACCESSED)
> > > > - __folio_set_referenced(folio);
> > > > + if (!mapping_large_folio_support(mapping))
> > > > + order = 0;
> > > > + if (order > MAX_PAGECACHE_ORDER)
> > > > + order = MAX_PAGECACHE_ORDER;
> > > > + /* If we're not aligned, allocate a smaller folio */
> > > > + if (index & ((1UL << order) - 1))
> > > > + order = __ffs(index);
> > >
> > > If I read this right, if we pass in an unaligned index, we won't get
> > > the size of the folio we ask for?
> >
> > Right. That's implied by (but perhaps not obvious from) the changelog.
> > Folios are always naturally aligned in the file, so an order-4 folio
> > has to start at a multiple of 16. If the index you pass in is not
> > a multiple of 16, we can't create an order-4 folio without starting
> > at an earlier index.
> >
> > For a 4kB block size filesystem, that's what we want. Applications
> > _generally_ don't write backwards, so creating an order-4 folio is just
> > wasting memory.
> >
> > > e.g. if we want an order-4 folio (64kB) because we have a 64kB block
> > > size in the filesystem, then we have to pass in an index that
> > > order-4 aligned, yes?
> > >
> > > I ask this, because the later iomap code that asks for large folios
> > > only passes in "pos >> PAGE_SHIFT" so it looks to me like it won't
> > > allocate large folios for anything other than large folio aligned
> > > writes, even if we need them.
> > >
> > > What am I missing?
> >
> > Perhaps what you're missing is that this isn't trying to solve the
> > problem of supporting a bs > ps filesystem?
>
> No, that's not what I'm asking about. I know there's other changes
> needed to enforce minimum folio size/alignment for bs > ps.

OK. Bringing up the 64kB block size filesystem confused me.

> What I'm asking about is when someone does a 16kB write at offset
> 12kB, they won't get a large folio allocated at all, right? Even
> though the write is large enough to enable it?

Right.

> Indeed, if we do a 1MB write at offset 4KB, we'll get 4kB at 4KB, 8KB
> and 12kB (because we can't do order-1 folios), then order-2 at 16KB,
> order-3 at 32kB, and so on until we hit offset 1MB where we will do
> an order-0 folio allocation again (because the remaining length is
> 4KB). The next 1MB write will then follow the same pattern, right?

Yes. Assuming we get another write ...

> I think this ends up being sub-optimal and fairly non-obvious
> non-obvious behaviour from the iomap side of the fence which is
> clearly asking for high-order folios to be allocated. i.e. a small
> amount of allocate-around to naturally align large folios when the
> page cache is otherwise empty would make a big difference to the
> efficiency of non-large-folio-aligned sequential writes...

At this point we're arguing about what I/O pattern to optimise for.
I'm going for a "do no harm" approach where we only allocate exactly as
much memory as we did before. You're advocating for a
higher-risk/higher-reward approach.

I'd prefer the low-risk approach for now; we can change it later!
I'd like to see some amount of per-fd write history (as we have per-fd
readahead history) to decide whether to allocate large folios ahead of
the current write position. As with readahead, I'd like to see that even
doing single-byte writes can result in the allocation of large folios,
as long as the app has done enough of them.