Having worked on slides and done further research in this area (I plan to really attack anon_vma over the coming year it's a real area of interest of mine), I have decided to modify the topic a little. Rather than focusing on the possible future ideal very-long-term project of finding a means of unifying anon + file-backed handling, I'd like to take a step back and discuss anon_vma in general and then examine: - short term improvements that I intend to attack shortly (hopefully some of which I will have submitted patches for -prior to lsf- as some people are apparently adament one should only speak about things one has patched). - medium term improvements that require architectural changes to the anon_vma mechanism. - and long term improvements which is, yes, unifying anon_vma and file-backed mappings. I think this will be more practical and we'll get a better more actionable discussion out of this approach. On Wed, Jan 08, 2025 at 10:23:16PM +0000, Lorenzo Stoakes wrote: > Hi all, > > Since time immemorial the kernel has maintained two separate realms within > mm - that of file-backed mappings and that of anonymous mappings. > > Each of these require a reverse mapping from folio to VMA, utilising > interval trees from an intermediate object referenced by folio->mapping > back to the VMAs which map it. > > In the case of a file-backed mapping, this 'intermediate object' is the > shared page cache entry, of type struct address_space. It is non-CoW which > keep things simple(-ish) and the concept is straight-forward - both the > folio and the VMAs which map the page cache object reference it. > > In the case of anonymous memory, things are not quite as simple, as a > result of CoW. This is further complicated by forking and the very many > different combinations of CoW'd and non-CoW'd folios that can exist within > a mapping. > > This kind of mapping utilises struct anon_vma objects which as a result of > this complexity are pretty well entirely concerned with maintaining the > notion of an anon_vma object rather than describing the underlying memory > in any way. > > Of course we can enter further realms of insan^W^W^W^W^Wcomplexity by > maintaining a MAP_PRIVATE file-backed mapping where we can experience both > at once! > > The fact that we can have both CoW'd and non-CoW'd folios referencing a VMA > means that we require -yet another- type, a struct anon_vma_chain, > maintained on a linked list, to abstract the link between anon_vma objects > and VMAs, and to provide a means by which one can manage and traverse > anon_vma objects from the VMA as well as looking them up from the reverse > mapping. > > Maintaining all of this correctly is very fragile, error-prone and > confusing, not to mention the concerns around maintaining correct locking > semantics, correctly propagating anonymous VMA state on fork, and trying to > reuse state to avoid allocating unnecessary memory to maintain all of this > infrastructure. > > An additional consequence of maintaining these two realms is that that > which straddles them - shmem - becomes something of an enigma - > file-backed, but existing on the anonymous LRU list and requiring a lot of > very specific handling. > > It is obvious that there is some isomorphism between the representation of > file systems and anonymous memory, less the CoW handling. However there is > a concept which exists within file systems which can somewhat bridge the gap > - reflinks. > > A future where we unify anonymous and file-backed memory mappings would be > one in which a reflinks were implemented at a general level rather than, as > they are now, implemented individually within file systems. > > I'd like to discuss how feasible doing so might be, whether this is a sane > line of thought at all, and how a roadmap for working towards the > elimination of anon_vma as it stands might look. > > As with my other proposal, I will gather more concrete information before > LSF to ensure the discussion is specific, and of course I would be > interested to discuss the topic in this thread also! > > Thanks!
