On Mon, Jan 23, 2023 at 12:00 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Mon 23-01-23 19:30:43, Matthew Wilcox wrote: > > On Mon, Jan 23, 2023 at 08:18:37PM +0100, Michal Hocko wrote: > > > On Mon 23-01-23 18:23:08, Matthew Wilcox wrote: > > > > On Mon, Jan 23, 2023 at 09:46:20AM -0800, Suren Baghdasaryan wrote: > > > [...] > > > > > Yes, batching the vmas into a list and draining it in remove_mt() and > > > > > exit_mmap() as you suggested makes sense to me and is quite simple. > > > > > Let's do that if nobody has objections. > > > > > > > > I object. We *know* nobody has a reference to any of the VMAs because > > > > you have to have a refcount on the mm before you can get a reference > > > > to a VMA. If Michal is saying that somebody could do: > > > > > > > > mmget(mm); > > > > vma = find_vma(mm); > > > > lock_vma(vma); > > > > mmput(mm); > > > > vma->a = b; > > > > unlock_vma(mm, vma); > > > > > > > > then that's something we'd catch in review -- you obviously can't use > > > > the mm after you've dropped your reference to it. > > > > > > I am not claiming this is possible now. I do not think we want to have > > > something like that in the future either but that is really hard to > > > envision. I am claiming that it is subtle and potentially error prone to > > > have two different ways of mass vma freeing wrt. locking. Also, don't we > > > have a very similar situation during last munmaps? > > > > We shouldn't have two ways of mass VMA freeing. Nobody's suggesting that. > > There are two cases; there's munmap(), which typically frees a single > > VMA (yes, theoretically, you can free hundreds of VMAs with a single > > call which spans multiple VMAs, but in practice that doesn't happen), > > and there's exit_mmap() which happens on exec() and exit(). > > This requires special casing remove_vma for those two different paths > (exit_mmap and remove_mt). If you ask me that sounds like a suboptimal > code to even not handle potential large munmap which might very well be > a rare thing as you say. But haven't we learned that sooner or later we > will find out there is somebody that cares afterall? Anyway, this is not > something I care about all that much. It is just weird to special case > exit_mmap, if you ask me. Up to Suren to decide which way he wants to > go. I just really didn't like the initial implementation of batching > based on a completely arbitrary batch limit and lazy freeing. I would prefer to go with the simplest sufficient solution. A potential issue with a large munmap might prove to be real but I think we know how to easily fix that with batching if the issue ever materializes (I'll have a fix ready implementing Michal's suggestion). So, I suggest going with Liam's/Matthew's solution and converting to Michal's solution if regression shows up anywhere else. Would that be acceptable? > -- > Michal Hocko > SUSE Labs
