Re: [PATCH v2 5/5] mm/madvise: eliminate very confusing manipulation of prev VMA

[Zi Yan]

On 20 Jun 2025, at 11:33, Lorenzo Stoakes wrote:

> The madvise code has for the longest time had very confusing code around
> the 'prev' VMA pointer passed around various functions which, in all cases
> except madvise_update_vma(), is unused and instead simply updated as soon
> as the function is invoked.
>
> To compound the confusion, the prev pointer is also used to indicate to the
> caller that the mmap lock has been dropped and that we can therefore not
> safely access the end of the current VMA (which might have been updated by
> madvise_update_vma()).
>
> Clear up this confusion by not setting prev = vma anywhere except in
> madvise_walk_vmas(), update all references to prev which will always be
> equal to vma after madvise_vma_behavior() is invoked, and adding a flag to
> indicate that the lock has been dropped to make this explicit.
>
> Additionally, drop a redundant BUG_ON() from madvise_collapse(), which is
> simply reiterating the BUG_ON(mmap_locked) above it (note that BUG_ON() is
> not appropriate here, but we leave existing code as-is).
>
> We finally adjust the madvise_walk_vmas() logic to be a little clearer -
> delaying the assignment of the end of the range to the start of the new
> range until the last moment and handling the lock being dropped scenario
> immediately.
>
> Additionally add some explanatory comments.
>
> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> ---
>  include/linux/huge_mm.h |  9 +++--
>  mm/khugepaged.c         |  9 ++---
>  mm/madvise.c            | 77 +++++++++++++++++++++--------------------
>  3 files changed, 47 insertions(+), 48 deletions(-)
>
Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

--
Best Regards,
Yan, Zi