On Wed, Mar 12, 2025 at 09:06:02AM -0700, Suren Baghdasaryan wrote: > > Any file or anonymous folio can be temporarily pinned for I/O and only > > moved once that completes. Direct I/O is one use case for that but there > > are plenty others. I'm not sure how you define "beforehand", but the > > pinning is visible in the _pincount field. > > Well, by "beforehand" I mean that when allocating for Direct I/O > operation we know this memory will be pinned, Direct I/O is performed on anonymous (or more rarely) file backed pages that are allocated from the normal allocators. Some callers might know that they are eventually going to perform direct I/O on them, but most won't as that information is a few layers removed from them or totally hidden in libraries. The same is true for other pin_user_pages operations. If you want memory that is easily available for CMA allocations it better not be given out as anonymous memory, and probably also not as file backed memory. Which just leaves you with easily migratable kernel allocations, i.e. not much.
