Re: [Xen-devel] [PATCH] xenpaging: unify return value in nominate and evict

[Tim Deegan <tim@xxxxxxx>]

At 20:56 +0100 on 26 Jan (1327611377), Olaf Hering wrote:
> # HG changeset patch
> # User Olaf Hering <olaf@xxxxxxxxx>
> # Date 1327607757 -3600
> # Node ID ebc68ae15103061fa554745efb652b7d7e7b8e89
> # Parent  266a12304601226213a57e39cc11aa075acdfb58
> xenpaging: unify return value in nominate and evict
> 
> Let p2m_mem_paging_nominate and p2m_mem_paging_evict return just one
> error number. EINVAL is not very helpful in case of nominate, it can
> happen if the pager tries to nominate a ballooned page. In this case the
> gfn is not backed by a mfn, the pager can not know that.  Similar with
> evict, anything can happen between nominate and evict.
> 
> This change helps the pager to decide if the returned error is from the
> function itself, or if it happend earlier. In the latter case, it is
> most likely fatal and should be handled as such.
> nominate returns EAGAIN, evict EBUSY.
> 
> Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
> 
> diff -r 266a12304601 -r ebc68ae15103 xen/arch/x86/mm/p2m.c
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -735,19 +735,17 @@ int p2m_mem_paging_nominate(struct domai
>      p2m_type_t p2mt;
>      p2m_access_t a;
>      mfn_t mfn;
> -    int ret;
> +    int ret = -EAGAIN;
>  
>      p2m_lock(p2m);
>  
>      mfn = p2m->get_entry(p2m, gfn, &p2mt, &a, p2m_query, NULL);
>  
>      /* Check if mfn is valid */
> -    ret = -EINVAL;
>      if ( !mfn_valid(mfn) )
>          goto out;
>  

I don't think EAGAIN is the right thing to return here.  There's no
reason to think that retrying an invalid GFN will work next time. 

I'd be happy for these cases all to return the same code, but maybe
EINVAL would be a better choice. 

Cheers,

Tim.


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