On Sat, Feb 22, 2025 at 06:26:32PM +0000, Lorenzo Stoakes wrote: > 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. I'm going to further refine this to discussing anon_vma in the context of VMA merging solely, and leave the bigger stuff to later :) This is in relation to an RFC patch series I have been working on. > > 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!
