On Thu, Feb 8, 2024 at 2:36 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > Kairui Song <ryncsn@xxxxxxxxx> writes: > > > On Thu, Feb 8, 2024 at 2:31 AM Minchan Kim <minchan@xxxxxxxxxx> wrote: > >> > >> On Wed, Feb 07, 2024 at 12:06:15PM +0800, Kairui Song wrote: > > [snip] > > >> > > >> > So I think the thing is, it's getting complex because this patch > >> > wanted to make it simple and just reuse the swap cache flags. > >> > >> I agree that a simple fix would be the important at this point. > >> > >> Considering your description, here's my understanding of the other idea: > >> Other method, such as increasing the swap count, haven't proven effective > >> in your tests. The approach risk forcing racers to rely on the swap cache > >> again and the potential performance loss in race scenario. > >> > >> While I understand that simplicity is important, and performance loss > >> in this case may be infrequent, I believe swap_count approach could be a > >> suitable solution. What do you think? > > > > Hi Minchan > > > > Yes, my main concern was about simplicity and performance. > > > > Increasing swap_count here will also race with another process from > > releasing swap_count to 0 (swapcache was able to sync callers in other > > call paths but we skipped swapcache here). > > What is the consequence of the race condition? Hi Ying, It will increase the swap count of an already freed entry, this race with multiple swap free/alloc logic that checks if count == SWAP_HAS_CACHE or sets count to zero, or repeated free of an entry, all result in random corruption of the swap map. This happens a lot during stress testing. > > > So the right step is: 1. Lock the cluster/swap lock; 2. Check if still > > have swap_count == 1, bail out if not; 3. Set it to 2; > > __swap_duplicate can be modified to support this, it's similar to > > existing logics for SWAP_HAS_CACHE. > > > > And swap freeing path will do more things, swapcache clean up needs to > > be handled even in the bypassing path since the racer may add it to > > swapcache. > > > > Reusing SWAP_HAS_CACHE seems to make it much simpler and avoided many > > overhead, so I used that way in this patch, the only issue is > > potentially repeated page faults now. > > > > I'm currently trying to add a SWAP_MAP_LOCK (or SWAP_MAP_SYNC, I'm bad > > at naming it) special value, so any racer can just spin on it to avoid > > all the problems, how do you think about this? > > Let's try some simpler method firstly. Another simpler idea is, add a schedule() or schedule_timeout_uninterruptible(1) in the swapcache_prepare failure path before goto out (just like __read_swap_cache_async). I think this should ensure in almost all cases, PTE is ready after it returns, also yields more CPU.
