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	Tue, 11 Feb 2025 00:15:21 +0000
Date: Tue, 11 Feb 2025 00:15:21 +0000
From: Al Viro <viro@zeniv.linux.org.uk>
To: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Cc: "idryomov@gmail.com" <idryomov@gmail.com>,
	Alex Markuze <amarkuze@redhat.com>,
	"slava@dubeyko.com" <slava@dubeyko.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"ceph-devel@vger.kernel.org" <ceph-devel@vger.kernel.org>,
	Patrick Donnelly <pdonnell@redhat.com>
Subject: Re: [PATCH] ceph: is_root_ceph_dentry() cleanup
Message-ID: <20250211001521.GF1977892@ZenIV>
References: <20250128011023.55012-1-slava@dubeyko.com>
 <20250128030728.GN1977892@ZenIV>
 <dfafe82535b7931e99790a956d5009a960dc9e0d.camel@ibm.com>
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Tue, Feb 11, 2025 at 12:08:21AM +0000, Viacheslav Dubeyko wrote:

> In general, the MDS can issue NULL dentries to clients so that  they "know" the
> direntry does not exist without having some capability (or lease) associated
> with it. As far as I can see, if application repeatedly does stat of file, then
> the kernel driver isn't repeatedly going out to the MDS to lookup that file. So,
> I assume that this is the goal of this check and logic.

Er...  On repeated stat(2) you will get ->lookup() called the first time around;
afterwards you'll be getting dentry from dcache lookup.  With ->d_revalidate()
each time you find it in dcache, and eviction if ->d_revalidate() says it's stale.
In which case a new dentry will be allocated and fed to ->lookup(), passed to
it in negative-unhashed state...