On Wed, May 6, 2020 at 6:29 PM Jan Kara jack@suse.cz wrote:
On Wed 06-05-20 17:51:39, Souptick Joarder wrote:
On Wed, May 6, 2020 at 3:36 PM Jan Kara jack@suse.cz wrote:
On Wed 06-05-20 02:06:56, Souptick Joarder wrote:
On Wed, May 6, 2020 at 1:08 AM John Hubbard jhubbard@nvidia.com wrote:
On 2020-05-05 12:14, Souptick Joarder wrote:
Currently {get|pin}_user_pages_fast() have 3 return value 0, -errno and no of pinned pages. The only case where these two functions will return 0, is for nr_pages <= 0, which doesn't find a valid use case. But if at all any, then a -ERRNO will be returned instead of 0, which means {get|pin}_user_pages_fast() will have 2 return values -errno & no of pinned pages.
Update all the callers which deals with return value 0 accordingly.
Hmmm, seems a little shaky. In order to do this safely, I'd recommend first changing gup_fast/pup_fast so so that they return -EINVAL if the caller specified nr_pages==0, and of course auditing all callers, to ensure that this won't cause problems.
While auditing it was figured out, there are 5 callers which cares for return value 0 of gup_fast/pup_fast. What problem it might cause if we change gup_fast/pup_fast to return -EINVAL and update all the callers in a single commit ?
Well, first I'd ask a different question: Why do you want to change the current behavior? It's not like the current behavior is confusing. Callers that pass >0 pages can happily rely on the simple behavior of < 0 return on error or > 0 return if we mapped some pages. Callers that can possibly ask to map 0 pages can get 0 pages back - kind of expected - and I don't see any benefit in trying to rewrite these callers to handle -EINVAL instead...
Callers with a request to map 0 pages doesn't have a valid use case. But if any caller end up doing it mistakenly, -errno should be returned to caller rather than 0 which will indicate more precisely that map 0 pages is not a valid request from caller.
Well, I believe this depends on the point of view. Similarly as reading 0 bytes is successful, we could consider mapping 0 pages successful as well. And there can be valid cases where number of pages to map is computed from some input and when 0 pages should be mapped, it is not a problem and your change would force such callers to special case this with explicitely checking for 0 pages to map and not calling GUP in that case at all.
I'm not saying what you propose is necessarily bad, I just say I don't find it any better than the current behavior and so IMO it's not worth the churn. Now if you can come up with some examples of current in-kernel users who indeed do get the handling of the return value wrong, I could be convinced otherwise.
There are 5 callers of {get|pin}_user_pages_fast().
arch/ia64/kernel/err_inject.c#L145 staging/gasket/gasket_page_table.c#L489
Checking return value 0 doesn't make sense for above 2.
drivers/platform/goldfish/goldfish_pipe.c#L277 net/rds/rdma.c#L165 drivers/tee/tee_shm.c#L262
These 3 callers have calculated the no of pages value before passing it to {get|pin}_user_pages_fast(). But if they end up passing nr_pages <= 0, a return value of either 0 or -EINVAL doesn't going to harm any existing behavior of callers.
IMO, it is safe to return -errno for nr_pages <= 0, for {get|pin}_user_pages_fast().