On Mon, Apr 15, 2024 at 9:13 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Mon, Apr 15, 2024 at 08:58:28AM -0700, Suren Baghdasaryan wrote: > > On Fri, Apr 12, 2024 at 8:19 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > On Fri, Apr 12, 2024 at 09:53:29AM -0500, Suren Baghdasaryan wrote: > > > > Unless vmf_anon_prepare() already explains why vma->anon_vma poses a > > > > problem for per-vma locks, we should have an explanation there. This > > > > comment would serve that purpose IMO. > > > > > > I'll do you one better; here's some nice kernel-doc for > > > vmd_anon_prepare(): > > > > I was looking at the find_tcp_vma(), which seems to be the only other > > place where lock_vma_under_rcu() is currently used. I think it's used > > there only for file-backed pages, so I don't think your change affects > > that usecase but this makes me think that we should have some kind of > > a warning for lock_vma_under_rcu() future users... Maybe your addition > > of mmap_assert_locked() inside __anon_vma_prepare() is enough. Please > > don't forget to include that assertion into your final patch. > > That's patch 1/3 on the git branch I pointed you to. Ah, good! > > The tcp vma is not file backed, but I'm pretty sure that COW is not > something they want, so there's never an anon_vma. It's for pages > that contain received TCP packets; ie it's mmaped TCP. I was following tcp_zerocopy_receive()->tcp_zerocopy_vm_insert_batch()->vm_insert_pages()->insert_page_in_batch_locked()->validate_page_before_insert() which errors out for PageAnon(page). So, I assumed this path works on file-backed pages but I'm not familiar with this code at all.
