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 WLw8sTqS 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wrote: > On Fri, Aug 08, 2025 at 04:19:09PM +0200, Vlastimil Babka wrote: > > On 8/3/25 13:11, Lorenzo Stoakes wrote: > [snip] > > > Signed-off-by: Lorenzo Stoakes > > > --- > > > mm/mremap.c | 20 ++++++++++++-------- > > > 1 file changed, 12 insertions(+), 8 deletions(-) > [snip] > > > @@ -1863,9 +1861,14 @@ static unsigned long remap_move(struct vma_remap_struct *vrm) > > > vrm->new_addr = target_addr + offset; > > > vrm->old_len = vrm->new_len = len; > > > > > > - allowed = vma_multi_allowed(vma); > > > - if (seen_vma && !allowed) > > > - return -EFAULT; > > > + if (!vma_multi_allowed(vma)) { > > > + /* This is not the first VMA, abort immediately. */ > > > + if (seen_vma) > > > + return -EFAULT; > > > + /* This is the first, but there are more, abort. */ > > > + if (vma->vm_end < end) > > > + return -EFAULT; > > > > Hm there can just also be a gap, and we permit gaps at the end (unlike at > > the start), right? > > I don't think we should allow a single VMA with gap, it's actually more > correct to maintain existing behavour in this case. > > > So we might be denying a multi vma mremap for !vma_multi_allowed() > > reasons even if it's a single vma and a gap. > > This is therfore a useful exercise in preventing us from permitting this > case I think. > > > > > AFAICS this is not regressing the behavior prior to d23cb648e365 > > ("mm/mremap: permit mremap() move of multiple VMAs") as such mremap() would > > be denied anyway by the "/* We can't remap across vm area boundaries */" > > check in check_prep_vma(). > > Yup. > > And this code is _only_ called for MREMAP_FIXED. So nothing else is impacted. > > > > > So the question is just if we want this odd corner case to behave like this, > > and if yes then be more explicit about it perhaps. > > We definitely do IMO. There's no reason to change this behaviour. > > The end gap thing in multi was more a product of 'why not permit it' but > now is more a case of 'it means we don't have to go check or fail > partially'. > > So I think this is fine. > > > > > > + } > > > > > > res_vma = check_prep_vma(vrm); > > > if (!res_vma) > > > @@ -1874,7 +1877,8 @@ static unsigned long remap_move(struct vma_remap_struct *vrm) > > > return res_vma; > > > > > > if (!seen_vma) { > > > - VM_WARN_ON_ONCE(allowed && res_vma != new_addr); > > > + VM_WARN_ON_ONCE(vma_multi_allowed(vma) && > > > + res_vma != new_addr); > > > res = res_vma; > > > } > > > > > > > I can update the commit msg accordingly... I have asked Andrew to update with a clear explanation of this (see [0]), and made clear why I feel it's consistent for us to disallow this behaviour for non-eligible VMAs while permitting it for eligible ones. It means we can simply say 'for eligible pure moves, you may specify gaps between or after VMAs spanning 1 or more VMAs'. Cheers, Lorenzo [0]: https://lore.kernel.org/linux-mm/df80b788-0546-4b78-a2fa-64d26e5a35b8@lucifer.local/