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 The comment being dropped is incorrect since it's now out-of-date.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 <ta...@tklengyel.com>
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
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