Re: [LSF/MM/BPF TOPIC] Predictive readahead of dentries

[Amir Goldstein <amir73il@gmail.com>]

On Tue, Jan 14, 2025 at 4:38 AM Shyam Prasad N <nspmangalore@gmail.com> wrote:
>
> The Linux kernel does buffered reads and writes using the page cache
> layer, where the filesystem reads and writes are offloaded to the
> VM/MM layer. The VM layer does a predictive readahead of data by
> optionally asking the filesystem to read more data asynchronously than
> what was requested.
>
> The VFS layer maintains a dentry cache which gets populated during
> access of dentries (either during readdir/getdents or during lookup).
> This dentries within a directory actually forms the address space for
> the directory, which is read sequentially during getdents. For network
> filesystems, the dentries are also looked up during revalidate.
>
> During sequential getdents, it makes sense to perform a readahead
> similar to file reads. Even for revalidations and dentry lookups,
> there can be some heuristics that can be maintained to know if the
> lookups within the directory are sequential in nature. With this, the
> dentry cache can be pre-populated for a directory, even before the
> dentries are accessed, thereby boosting the performance. This could
> give even more benefits for network filesystems by avoiding costly
> round trips to the server.
>

I believe you are referring to READDIRPLUS, which is quite common
for network protocols and also supported by FUSE.

Unlike network protocols, FUSE decides by server configuration and
heuristics whether to "fuse_use_readdirplus" - specifically in readdirplus_auto
mode, FUSE starts with readdirplus, but if nothing calls lookup on the
directory inode by the time the next getdents call, it stops with readdirplus.

I personally ran into the problem that I would like to control from the
application, which knows if it is doing "ls" or "ls -l" whether a specific
getdents() will use FUSE readdirplus or not, because in some situations
where "ls -l" is not needed that can avoid a lot of unneeded IO.

I do not know if implementing readdirplus (i.e. populate inode and dentry)
makes sense for disk filesystems, but if we do it in VFS level, there has to
be at an API to control or at least opt-out of readdirplus, like with readahead.

Thanks,
Amir.