On Wed, Dec 11, 2024 at 04:50:39PM +0000, Matthew Wilcox wrote: > On Wed, Dec 11, 2024 at 11:09:56AM -0500, Johannes Weiner wrote: > > This would work, but it seems somewhat complicated. The atomics in > > memcg charging and the vmstat updates are batched, and the per-page > > overhead is for the most part cheap per-cpu ops. Not an issue per se. > > OK, fair enough, I hadn't realised it was a percpu-refcount. Still, > we might consume several batches (batch size of 64) when we could do it > all in one shot. > > Perhaps you'd be more persuaded by: > > (a) If we clear __GFP_ACCOUNT then alloc_pages_bulk() will work, and > that's a pretty significant performance win over calling alloc_pages() > in a loop. > > (b) Once we get to memdescs, calling alloc_pages() with __GFP_ACCOUNT > set is going to require allocating a memdesc to store the obj_cgroup > in, so in the future we'll save an allocation. > > Your proposed alternative will work and is way less churn. But it's > not preparing us for memdescs ;-) We can make alloc_pages_bulk() work with __GFP_ACCOUNT but your second argument is more compelling. I am trying to think of what will we miss if we remove this per-page memcg metadata. One thing I can think of is debugging a live system or kdump where I need to track where a given page came from. I think memory profiling will still be useful in combination with going through all vmalloc regions where this page is mapped (is there an easy way to tell if a page is from a vmalloc region?). So, for now I think we will have alternative way to extract the useful information. I think we can go with Johannes' solution for stable and discuss the future direction more separately.
