linux-kernel - Re: [PATCH v6 1/9] fs: add infrastructure for multigrain timestamps

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Message-ID: <796973ee97c0b16761b41f510b852725da07eed7.camel@kernel.org>
Date: Thu, 08 Aug 2024 20:00:44 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner
 <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 linux-fsdevel@...r.kernel.org,  linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/9] fs: add infrastructure for multigrain timestamps

On Fri, 2024-08-09 at 00:09 +0200, Mateusz Guzik wrote:
> On Mon, Jul 15, 2024 at 08:48:52AM -0400, Jeff Layton wrote:
> > diff --git a/fs/stat.c b/fs/stat.c
> > index 6f65b3456cad..df7fdd3afed9 100644
> > --- a/fs/stat.c
> > +++ b/fs/stat.c
> > @@ -26,6 +26,32 @@
> >  #include "internal.h"
> >  #include "mount.h"
> >  
> > +/**
> > + * fill_mg_cmtime - Fill in the mtime and ctime and flag ctime as QUERIED
> > + * @stat: where to store the resulting values
> > + * @request_mask: STATX_* values requested
> > + * @inode: inode from which to grab the c/mtime
> > + *
> > + * Given @inode, grab the ctime and mtime out if it and store the result
> > + * in @stat. When fetching the value, flag it as queried so the next write
> > + * will ensure a distinct timestamp.
> > + */
> > +void fill_mg_cmtime(struct kstat *stat, u32 request_mask, struct inode *inode)
> > +{
> > +	atomic_t *pcn = (atomic_t *)&inode->i_ctime_nsec;
> > +
> > +	/* If neither time was requested, then don't report them */
> > +	if (!(request_mask & (STATX_CTIME|STATX_MTIME))) {
> > +		stat->result_mask &= ~(STATX_CTIME|STATX_MTIME);
> > +		return;
> > +	}
> > +
> > +	stat->mtime = inode_get_mtime(inode);
> > +	stat->ctime.tv_sec = inode->i_ctime_sec;
> > +	stat->ctime.tv_nsec = ((u32)atomic_fetch_or(I_CTIME_QUERIED, pcn)) & ~I_CTIME_QUERIED;
> > +}
> > +EXPORT_SYMBOL(fill_mg_cmtime);
> > +
> 
> [trimmed the ginormous CC]
> 
> This performs the atomic every time (as in it sets the flag even if it
> was already set), serializing all fstats and reducing scalability of
> stat of the same file.
> 
> Bare minimum it should be conditional -- if the flag is already set,
> don't dirty anything.
> 
> Even that aside adding an atomic to stat is a bummer, but off hand I
> don't have a good solution for that.
> 
> Anyhow, this being in -next, perhaps the conditional dirty can be
> massaged into the thing as present? There are some cosmetic choices to
> be made how to express, may be the fastest if you guys just augment it
> however you see fit.
> 
> If not I can submit a patch tomorrow.

I'm fine with that change. It should simple enough.
-- 
Jeff Layton <jlayton@...nel.org>