Messages in this thread | | | From | Brian Geffon <> | Date | Tue, 16 Mar 2021 15:18:55 -0400 | Subject | Re: [PATCH] mm: Allow shmem mappings with MREMAP_DONTUNMAP |
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Hi Hugh, Thanks for this suggestion, responses in line.
> A better patch would say: > > - if (flags & MREMAP_DONTUNMAP && (!vma_is_anonymous(vma) || > - vma->vm_flags & VM_SHARED)) > + if ((flags & MREMAP_DONTUNMAP) && > + (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))) > return ERR_PTR(-EINVAL); > > VM_DONTEXPAND is what has long been used on special mappings, to prevent > surprises from mremap changing the size of the mapping: MREMAP_DONTUNMAP > introduced a different way of expanding the mapping, so VM_DONTEXPAND > still seems a reasonable name (I've thrown in VM_PFNMAP there because > it's in the VM_DONTEXPAND test lower down: for safety I guess, and best > if both behave the same - though one says -EINVAL and the other -EFAULT).
I like this idea and am happy to mail a new patch. I think it may make sense to bring the lower block up here so that it becomes more clear that it's not duplicate code and that the MREMAP_DONTUNMAP case returns -EINVAL and other cases return -EFAULT. I wonder if the -EFAULT error code would have made more sense from the start for both cases, do you have any thoughts on changing the error code at this point?
> With that VM_DONTEXPAND check in, Dmitry's commit cd544fd1dc92 > ("mremap: don't allow MREMAP_DONTUNMAP on special_mappings and aio") > can still be reverted (as you agreed on 28th December), even though > vma_is_anonymous() will no longer protect it.
I agree and if Dmitry does not have time I would be happy to mail a revert to cd544fd1dc92 as we discussed in [1]. Dmitry, would you like me to do that?
> Was there an mremap(2) man page update for MREMAP_DONTUNMAP? > Whether or not there was before, it ought to get one now.
Yes, the mremap(2) man page was updated when this flag was added and it will require a further update to reflect this expanded mapping support.
Thanks Brian
1. https://lkml.org/lkml/2020/12/28/2340
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