On 10/29/22 13:30, Linus Torvalds wrote: >> I can think of three options: >> >> (a) filesystems just deal with it >> >> (b) we could move the "page_remove_rmap()" into the "flush-and-free" path too >> >> (c) we could actually add a spinlock (hashed on the page?) for this >> >> I think (a) is basically our current expectation. > > Side note: anybody doing gup + set_page_dirty() won't be fixed by b/c > anyway, so I think (a) is basically the only thing. > > And that's true even if you do a page pinning gup, since the source of > the gup may be actively unmapped after the gup. I was just now writing a response that favored (c) over (b), precisely because of that, yes. :) > > So a filesystem that thinks that only write, or a rmap-accessible mmap > can turn the page dirty really seems to be fundamentally broken. > > And I think that has always been the case, it's just that filesystem > writers may not have been happy with it, and may not have had > test-cases for it. > > It's not surprising that the filesystem people then try to blame users. > > Linus Yes, lots of unhappy debates about this over the years. However, I remain intrigued by (c), because if we had a "dirty page lock" that is looked up by page (much like looking up the ptl), it seems like a building block that would potentially help solve the whole thing. The above points about "file system needs to coordinate with mm about what's allowed to be dirtied, including gup/dma cases", those are still true and not yet solved, yes. But having a solid point of synchronization for this, definitely looks interesting. Of course, without working through this more thoroughly, it's not fair to impose this constraint on the current discussion, understood. :) thanks, -- John Hubbard NVIDIA
