Re: [Xen-devel] [PATCH v6 1/5] x86/mem_sharing: reorder when pages are unlocked and released

[Jan Beulich <JBeulich@xxxxxxxx>]

On 17.07.2019 21:33, Tamas K Lengyel wrote:
> Calling _put_page_type while also holding the page_lock
> for that page can cause a deadlock.
> 
> The comment being dropped is incorrect since it's now out-of-date.
> 
> Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>

The description covers ...

> --- a/xen/arch/x86/mm/mem_sharing.c
> +++ b/xen/arch/x86/mm/mem_sharing.c
> @@ -648,10 +648,6 @@ static int page_make_private(struct domain *d, struct 
> page_info *page)
>           return -EBUSY;
>       }
>   
> -    /* We can only change the type if count is one */
> -    /* Because we are locking pages individually, we need to drop
> -     * the lock here, while the page is typed. We cannot risk the
> -     * race of page_unlock and then put_page_type. */
>       expected_type = (PGT_shared_page | PGT_validated | PGT_locked | 2);
>       if ( page->u.inuse.type_info != expected_type )
>       {
> @@ -660,12 +656,11 @@ static int page_make_private(struct domain *d, struct 
> page_info *page)
>           return -EEXIST;
>       }
>   
> +    mem_sharing_page_unlock(page);
> +
>       /* Drop the final typecount */
>       put_page_and_type(page);
>   
> -    /* Now that we've dropped the type, we can unlock */
> -    mem_sharing_page_unlock(page);
> -
>       /* Change the owner */
>       ASSERT(page_get_owner(page) == dom_cow);
>       page_set_owner(page, d);

all of the above. But what about ...

> @@ -900,6 +895,7 @@ static int share_pages(struct domain *sd, gfn_t sgfn, 
> shr_handle_t sh,
>       p2m_type_t smfn_type, cmfn_type;
>       struct two_gfns tg;
>       struct rmap_iterator ri;
> +    unsigned long put_count = 0;
>   
>       get_two_gfns(sd, sgfn, &smfn_type, NULL, &smfn,
>                    cd, cgfn, &cmfn_type, NULL, &cmfn, 0, &tg);
> @@ -964,15 +960,6 @@ static int share_pages(struct domain *sd, gfn_t sgfn, 
> shr_handle_t sh,
>           goto err_out;
>       }
>   
> -    /* Acquire an extra reference, for the freeing below to be safe. */
> -    if ( !get_page(cpage, dom_cow) )
> -    {
> -        ret = -EOVERFLOW;
> -        mem_sharing_page_unlock(secondpg);
> -        mem_sharing_page_unlock(firstpg);
> -        goto err_out;
> -    }
> -
>       /* Merge the lists together */
>       rmap_seed_iterator(cpage, &ri);
>       while ( (gfn = rmap_iterate(cpage, &ri)) != NULL)
> @@ -984,13 +971,14 @@ static int share_pages(struct domain *sd, gfn_t sgfn, 
> shr_handle_t sh,
>            * Don't change the type of rmap for the client page. */
>           rmap_del(gfn, cpage, 0);
>           rmap_add(gfn, spage);
> -        put_page_and_type(cpage);
> +        put_count++;
>           d = get_domain_by_id(gfn->domain);
>           BUG_ON(!d);
>           BUG_ON(set_shared_p2m_entry(d, gfn->gfn, smfn));
>           put_domain(d);
>       }
>       ASSERT(list_empty(&cpage->sharing->gfns));
> +    BUG_ON(!put_count);
>   
>       /* Clear the rest of the shared state */
>       page_sharing_dispose(cpage);
> @@ -1001,7 +989,9 @@ static int share_pages(struct domain *sd, gfn_t sgfn, 
> shr_handle_t sh,
>   
>       /* Free the client page */
>       put_page_alloc_ref(cpage);
> -    put_page(cpage);
> +
> +    while ( put_count-- )
> +        put_page_and_type(cpage);
>   
>       /* We managed to free a domain page. */
>       atomic_dec(&nr_shared_mfns);
> @@ -1165,19 +1155,13 @@ int __mem_sharing_unshare_page(struct domain *d,
>       {
>           if ( !last_gfn )
>               mem_sharing_gfn_destroy(page, d, gfn_info);
> -        put_page_and_type(page);
> +
>           mem_sharing_page_unlock(page);
> +
>           if ( last_gfn )
> -        {
> -            if ( !get_page(page, dom_cow) )
> -            {
> -                put_gfn(d, gfn);
> -                domain_crash(d);
> -                return -EOVERFLOW;
> -            }
>               put_page_alloc_ref(page);
> -            put_page(page);
> -        }
> +
> +        put_page_and_type(page);
>           put_gfn(d, gfn);
>   
>           return 0;

... this (main, as I guess by the title) part of the change? I think
you want to explain what was wrong here and/or why the new arrangement
is better. (I'm sorry, I guess it was this way on prior versions
already, but apparently I didn't notice.)

Jan
_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel