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? > 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. -- Best Regards, Huang, Ying
