Re: [PATCH v1 2/2] xen: mapcache: Fix unmapping of first entries in buckets

[Alex Bennée <alex.bennee@xxxxxxxxxx>]

Anthony PERARD <anthony.perard@xxxxxxxxxx> writes:

> On Tue, Jul 02, 2024 at 12:44:21AM +0200, Edgar E. Iglesias wrote:
>> From: "Edgar E. Iglesias" <edgar.iglesias@xxxxxxx>
>> 
>> This fixes the clobbering of the entry->next pointer when
>> unmapping the first entry in a bucket of a mapcache.
>> 
>> Fixes: 123acd816d ("xen: mapcache: Unmap first entries in buckets")
>> Reported-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
>> Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xxxxxxx>
>> ---
>>  hw/xen/xen-mapcache.c | 12 +++++++++++-
>>  1 file changed, 11 insertions(+), 1 deletion(-)
>> 
>> diff --git a/hw/xen/xen-mapcache.c b/hw/xen/xen-mapcache.c
>> index 5f23b0adbe..18ba7b1d8f 100644
>> --- a/hw/xen/xen-mapcache.c
>> +++ b/hw/xen/xen-mapcache.c
>> @@ -597,7 +597,17 @@ static void 
>> xen_invalidate_map_cache_entry_unlocked(MapCache *mc,
>>          pentry->next = entry->next;
>>          g_free(entry);
>>      } else {
>> -        memset(entry, 0, sizeof *entry);
>> +        /*
>> +         * Invalidate mapping but keep entry->next pointing to the rest
>> +         * of the list.
>> +         *
>> +         * Note that lock is already zero here, otherwise we don't unmap.
>> +         */
>> +        entry->paddr_index = 0;
>> +        entry->vaddr_base = NULL;
>> +        entry->valid_mapping = NULL;
>> +        entry->flags = 0;
>> +        entry->size = 0;
>
> This kind of feels like mc->entry should be an array of pointer rather
> than an array of MapCacheEntry but that seems to work well enough and
> not the first time entries are been cleared like that.

The use of a hand rolled list is a bit of a concern considering QEMU and
Glib both provide various abstractions used around the rest of the code
base. The original patch that introduces the mapcache doesn't tell me
much about access patterns for the cache, just that it is trying to
solve memory exhaustion issues with lots of dynamic small mappings.

Maybe a simpler structure is desirable?

We also have an interval tree implementation ("qemu/interval-tree.h") if
what we really want is a sorted tree of memory that can be iterated
locklessly.

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro