linux-kernel - Re: [PATCH 1/3] mm/sparse: Clean up the obsolete code comment

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Message-ID: <20190320123747.vzreusrqx74zkdfm@d104.suse.de>
Date:   Wed, 20 Mar 2019 13:37:47 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Baoquan He <bhe@...hat.com>, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, pasha.tatashin@...cle.com,
        mhocko@...e.com, rppt@...ux.vnet.ibm.com,
        richard.weiyang@...il.com, linux-mm@...ck.org
Subject: Re: [PATCH 1/3] mm/sparse: Clean up the obsolete code comment

On Wed, Mar 20, 2019 at 05:22:43AM -0700, Matthew Wilcox wrote:
> On Wed, Mar 20, 2019 at 01:20:15PM +0100, Oscar Salvador wrote:
> > On Wed, Mar 20, 2019 at 04:19:59AM -0700, Matthew Wilcox wrote:
> > > On Wed, Mar 20, 2019 at 03:35:38PM +0800, Baoquan He wrote:
> > > >  /*
> > > > - * returns the number of sections whose mem_maps were properly
> > > > - * set.  If this is <=0, then that means that the passed-in
> > > > - * map was not consumed and must be freed.
> > > > + * sparse_add_one_section - add a memory section
> > > > + * @nid:	The node to add section on
> > > > + * @start_pfn:	start pfn of the memory range
> > > > + * @altmap:	device page map
> > > > + *
> > > > + * Return 0 on success and an appropriate error code otherwise.
> > > >   */
> > > 
> > > I think it's worth documenting what those error codes are.  Seems to be
> > > just -ENOMEM and -EEXIST, but it'd be nice for users to know what they
> > > can expect under which circumstances.
> > > 
> > > Also, -EEXIST is a bad errno to return here:
> > > 
> > > $ errno EEXIST
> > > EEXIST 17 File exists
> > > 
> > > What file?  I think we should be using -EBUSY instead in case this errno
> > > makes it back to userspace:
> > > 
> > > $ errno EBUSY
> > > EBUSY 16 Device or resource busy
> > 
> > We return -EEXIST in case the section we are trying to add is already
> > there, and that error is being caught by __add_pages(), which ignores the
> > error in case is -EXIST and keeps going with further sections.
> > 
> > Sure we can change that for -EBUSY, but I think -EEXIST makes more sense,
> > plus that kind of error is never handed back to userspace.
> 
> Not returned to userspace today.  It's also bad precedent for other parts
> of the kernel where errnos do get returned to userspace.

Yes, I get your point, but I do not really see -EBUSY fitting here.
Actually, we do have the same kind of situation when dealing with resources.
We return -EEXIST in register_memory_resource() in case the resource we are
trying to add conflicts with another one.

I think that -EEXIST is more intuitive in that code path, but I am not going to
insist.

-- 
Oscar Salvador
SUSE L3