Re: [PATCH] fuse: enable larger read buffers for readdir [v2].

[Jaco Kroon]

Hi,

On 2023/07/27 17:35, Miklos Szeredi wrote:
> On Thu, 27 Jul 2023 at 10:13, Jaco Kroon <jaco@xxxxxxxxx> wrote:
> > This patch does not mess with the caching infrastructure like the
> > previous one, which we believe caused excessive CPU and broke directory
> > listings in some cases.
> >
> > This version only affects the uncached read, which then during parse adds an
> > entry at a time to the cached structures by way of copying, and as such,
> > we believe this should be sufficient.
> >
> > We're still seeing cases where getdents64 takes ~10s (this was the case
> > in any case without this patch, the difference now that we get ~500
> > entries for that time rather than the 14-18 previously).  We believe
> > that that latency is introduced on glusterfs side and is under separate
> > discussion with the glusterfs developers.
> >
> > This is still a compile-time option, but a working one compared to
> > previous patch.  For now this works, but it's not recommended for merge
> > (as per email discussion).
> >
> > This still uses alloc_pages rather than kvmalloc/kvfree.
> >
> > Signed-off-by: Jaco Kroon <jaco@xxxxxxxxx>
> > ---
> >   fs/fuse/Kconfig   | 16 ++++++++++++++++
> >   fs/fuse/readdir.c | 18 ++++++++++++------
> >   2 files changed, 28 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/fuse/Kconfig b/fs/fuse/Kconfig
> > index 038ed0b9aaa5..0783f9ee5cd3 100644
> > --- a/fs/fuse/Kconfig
> > +++ b/fs/fuse/Kconfig
> > @@ -18,6 +18,22 @@ config FUSE_FS
> >            If you want to develop a userspace FS, or if you want to use
> >            a filesystem based on FUSE, answer Y or M.
> >
> > +config FUSE_READDIR_ORDER
> > +       int
> > +       range 0 5
> > +       default 5
> > +       help
> > +               readdir performance varies greatly depending on the size of the read.
> > +               Larger buffers results in larger reads, thus fewer reads and higher
> > +               performance in return.
> > +
> > +               You may want to reduce this value on seriously constrained memory
> > +               systems where 128KiB (assuming 4KiB pages) cache pages is not ideal.
> > +
> > +               This value reprents the order of the number of pages to allocate (ie,
> > +               the shift value).  A value of 0 is thus 1 page (4KiB) where 5 is 32
> > +               pages (128KiB).
> > +
> >   config CUSE
> >          tristate "Character device in Userspace support"
> >          depends on FUSE_FS
> > diff --git a/fs/fuse/readdir.c b/fs/fuse/readdir.c
> > index dc603479b30e..47cea4d91228 100644
> > --- a/fs/fuse/readdir.c
> > +++ b/fs/fuse/readdir.c
> > @@ -13,6 +13,12 @@
> >   #include <linux/pagemap.h>
> >   #include <linux/highmem.h>
> >
> > +#define READDIR_PAGES_ORDER            CONFIG_FUSE_READDIR_ORDER
> > +#define READDIR_PAGES                  (1 << READDIR_PAGES_ORDER)
> > +#define READDIR_PAGES_SIZE             (PAGE_SIZE << READDIR_PAGES_ORDER)
> > +#define READDIR_PAGES_MASK             (READDIR_PAGES_SIZE - 1)
> > +#define READDIR_PAGES_SHIFT            (PAGE_SHIFT + READDIR_PAGES_ORDER)
> > +
> >   static bool fuse_use_readdirplus(struct inode *dir, struct dir_context *ctx)
> >   {
> >          struct fuse_conn *fc = get_fuse_conn(dir);
> > @@ -328,25 +334,25 @@ static int fuse_readdir_uncached(struct file *file, struct dir_context *ctx)
> >          struct fuse_mount *fm = get_fuse_mount(inode);
> >          struct fuse_io_args ia = {};
> >          struct fuse_args_pages *ap = &ia.ap;
> > -       struct fuse_page_desc desc = { .length = PAGE_SIZE };
> > +       struct fuse_page_desc desc = { .length = READDIR_PAGES_SIZE };
> Does this really work?  I would've thought we are relying on single
> page lengths somewhere.
Based on my testing yes.  Getting just under 500 entries per 
getdents64() call from userspace vs 14-18 before I guess the answer is yes.
> >          u64 attr_version = 0;
> >          bool locked;
> >
> > -       page = alloc_page(GFP_KERNEL);
> > +       page = alloc_pages(GFP_KERNEL, READDIR_PAGES_ORDER);
> >          if (!page)
> >                  return -ENOMEM;
> >
> >          plus = fuse_use_readdirplus(inode, ctx);
> >          ap->args.out_pages = true;
> > -       ap->num_pages = 1;
> > +       ap->num_pages = READDIR_PAGES;
> No.  This is the array lenght, which is 1.  This is the hack I guess,
> which makes the above trick work.

Oh?  So the page referenced above isn't an array of pages?  It's 
actually a single piece of contiguous memory?

> Better use kvmalloc, which might have a slightly worse performance
> than a large page, but definitely not worse than the current single
> page.

Which returns a void*, not struct page* - guess this can be converted 
using __virt_to_page (iirc)?

> If we want to optimize the overhead of kvmalloc (and it's a big if)
> then the parse_dir*file() functions would need to be converted to
> using a page array instead of a plain kernel pointer, which would add
> some complexity for sure.

Sorry, I read the above as "I'm surprised this works at all and you're 
not kernel panicking all over the show", this is probably the most 
ambitious kernel patch I've attempted to date.

Kind regards,
Jaco