On 10/17/24 22:57, Jeff Xu wrote: > On Thu, Oct 17, 2024 at 1:49 PM Pedro Falcato <pedro.falcato@xxxxxxxxx> wrote: >> > >> > > > For file-backed, private, read-only memory mappings, we previously did >> > > > not block the madvise(MADV_DONTNEED). This was based on >> > > > the assumption that the memory's content, being file-backed, could be >> > > > retrieved from the file if accessed again. However, this assumption >> > > > failed to consider scenarios where a mapping is initially created as >> > > > read-write, modified, and subsequently changed to read-only. The newly >> > > > introduced VM_WASWRITE flag addresses this oversight. >> > > >> > > We *do not* need this. It's sufficient to just block discard operations on read-only >> > > private mappings. >> > I think you meant blocking madvise(MADV_DONTNEED) on all read-only >> > private file-backed mappings. >> > >> > I considered that option, but there is a use case for madvise on those >> > mappings that never get modified. >> > >> > Apps can use that to free up RAM. e.g. Considering read-only .text >> > section, which never gets modified, madvise( MADV_DONTNEED) can free >> > up RAM when memory is in-stress, memory will be reclaimed from a >> > backed-file on next read access. Therefore we can't just block all >> > read-only private file-backed mapping, only those that really need to, >> > such as mapping changed from rw=>r (what you described) >> >> Does anyone actually do this? If so, why? WHYYYY? >> > This is a legit use case, I can't argue that it isn't. Could the same effect be simply achieved with MADV_COLD/MADV_PAGEOUT? That should be able to reclaim the pages as well if they are indeed not used, but it's non-destructive and you don't want to allow destructive madvise anyway (i.e. no throwing away data that would be replaced by zeroes or original file content on the next touch) so it seems overall a better fit for sealed areas?
