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From:   Benjamin Coddington <bcodding@redhat.com>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     Trond Myklebust <trondmy@hammerspace.com>, anna@kernel.org,
        linux-nfs@vger.kernel.org
Subject: Re: [PATCH v2] NFSv4: Always ask for type with READDIR
Date:   Thu, 31 Aug 2023 11:17:35 -0400
Message-ID: <6BE9526F-51FE-4CD9-AE95-EE69F7F0CAA6@redhat.com>
In-Reply-To: <f4837c30b2faedd6a736a19d93c79b93df230349.camel@kernel.org>
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On 30 Aug 2023, at 17:14, Jeff Layton wrote:

> On Wed, 2023-08-30 at 20:20 +0000, Trond Myklebust wrote:
>> On Wed, 2023-08-30 at 16:10 -0400, Jeff Layton wrote:
>>> On Wed, 2023-08-30 at 15:42 -0400, Benjamin Coddington wrote:
>>>> Again we have claimed regressions for walking a directory tree,
>>>> this time
>>>> with the "find" utility which always tries to optimize away asking
>>>> for any
>>>> attributes until it has a complete list of entries.=C2=A0 This behav=
ior
>>>> makes
>>>> the readdir plus heuristic do the wrong thing, which causes a storm
>>>> of
>>>> GETATTRs to determine each entry's type in order to continue the
>>>> walk.
>>>>
>>>> For v4 add the type attribute to each READDIR request to include it
>>>> no
>>>> matter the heuristic.=C2=A0 This allows a simple `find` command to
>>>> proceed
>>>> quickly through a directory tree.
>>>>
>>>
>>> The important bit here is that with v4, we can fill out d_type even
>>> when
>>> "plus" is false, at little cost. The downside is that non-plus
>>> READDIR
>>> replies will now be a bit larger on the wire. I think it's a
>>> worthwhile
>>> tradeoff though.
>>
>> The reason why we never did it before is that for many servers, it
>> forces them to go to the inode in order to retrieve the information.
>>
>> IOW: You might as well just do readdirplus.
>>
>
> That makes total sense, given how this code has evolved.
>
> FWIW, the Linux NFS server already calls vfs_getattr for every dentry i=
n
> a v4 READDIR reply regardless of what the client requests. It has to in=

> order to detect junctions, so we're bringing in the inode no matter
> what. Fetching the type is trivial, so I don't see this as costing
> anything extra there.
>
> Mileage could vary on other servers with more synthetic filesystems, bu=
t
> one would hope that most of them can also return the type cheaply.

It occurred to me that we could let those other server folks ask for
whatever attributes they wanted if we make it tunable at runtime:

https://lore.kernel.org/linux-nfs/8f752f70daf73016e20c49508f825e8c2c94f5e=
7.1693494824.git.bcodding@redhat.com/T/#u

Ben