lkml.org 
[lkml]   [2021]   [Mar]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] mm: Allow shmem mappings with MREMAP_DONTUNMAP
From
Date
Hi Brian, Hugh,

On 3/16/21 7:18 PM, Brian Geffon wrote:
> 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?

Ack. I was planning to send a patches set that includes the revert, but
that's stalled a bit. As the patch just adds excessive checks, but
doesn't introduce an issue, I haven't sent it separately.
Feel free to revert it :-)

Thanks,
Dmitry

\
 
 \ /
  Last update: 2021-03-16 20:32    [W:0.086 / U:0.168 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site