Re: [PATCH] man2/openat2.2: add RESOLVE_CACHED

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Jens,

On 12/17/20 10:18 PM, Jens Axboe wrote:
> RESOLVE_CACHED allows an application to attempt a cache-only open
> of a file. If this isn't possible, the request will fail with
> -1/EAGAIN and the caller should retry without RESOLVE_CACHED set.
> This will generally happen from a different context, where a slower
> open operation can be performed.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

I take it that this feature is not yet merged into the kernel?
(At least,  I couldn't grep RESOLVE_CACHED in current
kernel sources.)

> 
> ---
> 
> First attempt at this, fully expecting the phrasing to require some
> massaging.
> 
> Also see: https://lore.kernel.org/linux-fsdevel/20201217161911.743222-1-axboe@kernel.dk/T/
> 
> diff --git a/man2/openat2.2 b/man2/openat2.2
> index 3bda20620574..282c13e2df96 100644
> --- a/man2/openat2.2
> +++ b/man2/openat2.2
> @@ -385,6 +385,15 @@ This may occur if, for example,
>  a system pathname that is used by an application is modified
>  (e.g., in a new distribution release)
>  so that a pathname component (now) contains a bind mount.
> +.TP
> +.B RESOLVE_CACHED
> +Fail path resolution, unless all components needed already exist in the

I think "Fail path resolution" needs some clarification.

Maybe something like:
[[
Make the open operation fail unless all path components
are already present in the kernel's lookup cache.
]]
?

> +lookup cache. If any kind of revalidation or IO is needed to satisfy the
                 ^
Please start new sentences on new lines. Using such-called "semantic
newlines" makes for cleaner future patches.

> +lookup, then fail the open attempt with

"fail the open attempt with"
==>
.BR openat2 ()
fails with the error

> +.B EAGAIN.
> +This is useful in providing a fast path open that can be performed without
> +resorting to thread offload, or other mechanism that an application might
> +use to offload slower operations.
>  .RE
>  .IP
>  If any bits other than those listed above are set in
> @@ -421,6 +430,14 @@ The caller may choose to retry the
>  .BR openat2 ()
>  call.
>  .TP
> +.B EAGAIN
> +.BR RESOLVE_CACHED
> +was set, and the open operation cannot be performed cached. The caller should

semantic newline

> +retry without
> +.B RESOLVE_CACHED
> +set in
> +.I how.resolve
> +.TP
>  .B EINVAL
>  An unknown flag or invalid value was specified in
>  .IR how .

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/