On Tue, Jun 27, 2023 at 01:56:09PM -0400, Liam R. Howlett wrote: [snip] > > > How about something like:- > > > > > > return find_vma_intersection(vma->mm, addr_masked, vma->vm_start) == NULL; > > > > > > Which explicitly asserts that the range in [addr_masked, vma->vm_start) is > > > empty. > > > > > > But actually, we should be able to go further and replace the previous > > > check with:- > > > > > > return find_vma_intersection(vma->mm, addr_masked, addr_to_align) == NULL; > > > > > > Which will fail if addr_to_align is offset within the VMA. > > > > Your suggestion would mean that we do a full VMA search starting from the > > root. That would not be a nice thing if say we've 1000s of VMAs? > > > > Actually Liam told me to use find_vma_prev() because given a VMA, the maple > > tree would not have to work that hard for the common case to find the > > previous VMA. Per conversing with him, there is a chance we may have to go > > one step above in the tree if we hit the edge of a node, but that's not > > supposed to be the common case. In previous code, the previous VMA could > > just be obtained using the "previous VMA" pointer, however that pointer has > > been remove since the maple tree changes and given a VMA, going to the > > previous one using the maple tree is just as fast (as I'm told). > > I think there's been a bit of a miscommunication on that.. > > If you have already found the VMA and are using the maple state, then > it's very little effort to get the next/prev. Leaf nodes can hold 16 > entries/NULL ranges, so the chances to go to the next/prev is usually in > the cpu cache already.. if you go up a level in the tree, then you will > have 10 nodes each with 16 entries each, etc, etc.. So the chances of > being on an edge node and having to walk up multiple levels to get to > the prev/next becomes rather rare.. and if you've just walked down, the > nodes on the way up will still be cached. > > Here, you're not using the maple state but searching for an address > using find_vma_prev(), but internally, that function does use a maple > state to get you the previous. So you are looking up the VMA from the > root, but the prev will very likely be in the CPU cache. > > Assuming the worst case tree (each VMA has a gap next to it, not really > going to happen as they tend to be grouped together), then we are > looking at a 4 level tree to get to 8,000 VMAs. 5 levels gets you a > minimum 80,000. I've never seen a tree of height 6 in the wild, but you > can fit 1.6M to 800K in one. > > I think the code is fine, but I wanted to clarify what we discussed. Would the same apply to find_vma_intersection(), as they equally searches from the root and allows the code to be made fairly succinct? I really am not a huge fan of find_vma_prev() searching for a VMA you already have just to get the previous one... would at lesat like to use vma_prev() on a newly defined vmi, but if find_vma_intersection() is fine then can reduce code to this. [snip]
