* Suren Baghdasaryan <surenb@xxxxxxxxxx> [230223 21:06]: > On Thu, Feb 23, 2023 at 5:46 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote: > > > > * Suren Baghdasaryan <surenb@xxxxxxxxxx> [230223 16:16]: > > > On Thu, Feb 23, 2023 at 12:28 PM Liam R. Howlett > > > <Liam.Howlett@xxxxxxxxxx> wrote: > > > > > > > > > > > > Wait, I figured a better place to do this. > > > > > > > > init_multi_vma_prep() should vma_start_write() on any VMA that is passed > > > > in.. that we we catch any modifications here & in vma_merge(), which I > > > > think is missed in this patch set? > > > > > > Hmm. That looks like a good idea but in that case, why not do the > > > locking inside vma_prepare() itself? From the description of that > > > function it sounds like it was designed to acquire locks before VMA > > > modifications, so would be the ideal location for doing that. WDYT? > > > > That might be even better. I think it will result in even less code. > > Yes. > > > > > There is also a vma_complete() which might work to call > > vma_end_write_all() as well? > > If there are other VMAs already locked before vma_prepare() then we > would unlock them too. Safer to just let mmap_unlock do > vma_end_write_all(). > > > > > > The only concern is vma_adjust_trans_huge() being called before > > > vma_prepare() but I *think* that's safe because > > > vma_adjust_trans_huge() does its modifications after acquiring PTL > > > lock, which page fault handlers also have to take. Does that sound > > > right? > > > > I am not sure. We are certainly safe the way it is, and the PTL has to > > be safe for concurrent faults.. but this could alter the walk to a page > > table while that walk is occurring and I don't think that happens today. > > > > It might be best to leave the locking order the way you have it, unless > > someone can tell us it's safe? > > Yes, I have the same feelings about changing this. > > > > > We could pass through the three extra variables that are needed to move > > the vma_adjust_trans_huge() call within that function as well? This > > would have the added benefit of having all locking grouped in the one > > location, but the argument list would be getting long, however we could > > use the struct. > > Any issues if I change the order to have vma_prepare() called always > before vma_adjust_trans_huge()? That way the VMA will always be locked > before vma_adjust_trans_huge() executes and we don't need any > additional arguments. I preserved the locking order from __vma_adjust() to ensure there was no issues. I am not sure but, looking through the page table information [1], it seems that vma_adjust_trans_huge() uses the pmd lock, which is part of the split page table lock. According to the comment in rmap, it should be fine to reverse the ordering here. Instead of: mmap_lock() vma_adjust_trans_huge() pte_lock pte_unlock vma_prepare() mapping->i_mmap_rwsem lock anon_vma->rwsem lock <changes to tree/VMAs> vma_complete() anon_vma->rwsem unlock mapping->i_mmap_rwsem unlock mmap_unlock() --------- We would have: mmap_lock() vma_prepare() mapping->i_mmap_rwsem lock anon_vma->rwsem lock vma_adjust_trans_huge() pte_lock pte_unlock <changes to tree/VMAs> vma_complete() anon_vma->rwsem unlock mapping->i_mmap_rwsem unlock mmap_unlock() Essentially, increasing the nesting of the pte lock, but not violating the ordering. 1. https://docs.kernel.org/mm/split_page_table_lock.html > > > > > remove & remove2 should be be detached in vma_prepare() or > > vma_complete() as well? > > They are marked detached in vma_complete() (see > https://lore.kernel.org/all/20230216051750.3125598-25-surenb@xxxxxxxxxx/) > and that should be enough. We should be safe as long as we mark them > detached before unlocking the VMA. > Right, Thanks. ...
