linux-kernel - Re: [PATCH v2 1/4] kernfs: kvmalloc xattr value instead of kmalloc

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Message-Id: <C16IH7NEXW4J.440OGTNY7CWX@dlxu-fedora-R90QNFJV>
Date:   Mon, 09 Mar 2020 11:21:33 -0700
From:   "Daniel Xu" <dxu@...uu.xyz>
To:     "Joe Perches" <joe@...ches.com>, <cgroups@...r.kernel.org>,
        <tj@...nel.org>, <lizefan@...wei.com>, <hannes@...xchg.org>
Cc:     <shakeelb@...gle.com>, <linux-kernel@...r.kernel.org>,
        <gregkh@...uxfoundation.org>, <kernel-team@...com>
Subject: Re: [PATCH v2 1/4] kernfs: kvmalloc xattr value instead of kmalloc

Hi Joe,

On Fri Mar 6, 2020 at 12:49 AM, Joe Perches wrote:
> On Thu, 2020-03-05 at 13:16 -0800, Daniel Xu wrote:
> > It's not really necessary to have contiguous physical memory for xattr
> > values. We no longer need to worry about higher order allocations
> > failing with kvmalloc, especially because the xattr size limit is at
> > 64K.
>
> 
> So why use vmalloc memory at all?
>
> 
> > diff --git a/fs/xattr.c b/fs/xattr.c
> ']
> > @@ -817,7 +817,7 @@ struct simple_xattr *simple_xattr_alloc(const void *value, size_t size)
> >  	if (len < sizeof(*new_xattr))
> >  		return NULL;
> >  
> > -	new_xattr = kmalloc(len, GFP_KERNEL);
> > +	new_xattr = kvmalloc(len, GFP_KERNEL);
>
> 
> Why is this sensible?
> vmalloc memory is a much more limited resource.

What would be the alternative? As Greg said, contiguous memory should be
more scarce.

> Also, it seems as if the function should set
> new_xattr->name to NULL before the return.
>

Will add and send in a different patch.


Thanks,
Daniel