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both a single VMA case, where we are > permitted to move a single VMA regardless of multi-VMA move eligiblity, and > multiple VMAs which, of course, must be eligible for such an operation. > > We determine this via vma_multi_allowed(). > > Currently, if the first VMA is not eligible, but others are, we will move > the first then return an error. This is not ideal, as we are performing an > operation which we don't need to do which has an impact on the memory > mapping. > > We can very easily determine if this is a multi VMA move prior to the move > of the first VMA, by checking vma->vm_end vs. the specified end address. > > Therefore this patch does so, and as a result eliminates unnecessary logic > around tracking whether the first VMA was permitted or not. > > This is most useful for cases where a user attempts to erroneously move > mutliple VMAs which are not eligible for non-transient reasons - for > instance, UFFD-armed VMAs, or file-backed VMAs backed by a file system or > driver which specifies a custom f_op->get_unmapped_area. > > In the less likely instance of a failure due to transient issues such as > out of memory or mapping limits being hit, the issue is already likely > fatal and so the fact the operation may be partially complete is > acceptable. > Previously, any attempt to solely move a VMA would require that the span specified reside within the span of that single VMA, with no gaps before or afterwards. After commit d23cb648e365 ("mm/mremap: permit mremap() move of multiple VMAs"), the multi VMA move permitted a gap to exist only after VMAs. This was done to provide maximum flexibility. However, We have consequently permitted this behaviour for the move of a single VMA including those not eligible for multi VMA move. The change introduced here means that we no longer permit non-eligible VMAs from being moved in this way. This is consistent, as it means all eligible VMA moves are treated the same, and all non-eligible moves are treated as they were before. This change does not break previous behaviour, which equally would have disallowed such a move (only in all cases). > Signed-off-by: Lorenzo Stoakes