Re: [PATCH v4 4/8] mm/rmap: Split migration into its own function

[Alistair Popple]

On Tuesday, 9 March 2021 5:58:12 AM AEDT Ralph Campbell wrote:
> 
> On 3/3/21 10:16 PM, Alistair Popple wrote:
> > Migration is currently implemented as a mode of operation for
> > try_to_unmap_one() generally specified by passing the TTU_MIGRATION flag
> > or in the case of splitting a huge anonymous page TTU_SPLIT_FREEZE.
> > 
> > However it does not have much in common with the rest of the unmap
> > functionality of try_to_unmap_one() and thus splitting it into a
> > separate function reduces the complexity of try_to_unmap_one() making it
> > more readable.
> > 
> > Several simplifications can also be made in try_to_migrate_one() based
> > on the following observations:
> > 
> >   - All users of TTU_MIGRATION also set TTU_IGNORE_MLOCK.
> >   - No users of TTU_MIGRATION ever set TTU_IGNORE_HWPOISON.
> >   - No users of TTU_MIGRATION ever set TTU_BATCH_FLUSH.
> > 
> > TTU_SPLIT_FREEZE is a special case of migration used when splitting an
> > anonymous page. This is most easily dealt with by calling the correct
> > function from unmap_page() in mm/huge_memory.c  - either
> > try_to_migrate() for PageAnon or try_to_unmap().
> > 
> > Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
> > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> 
> Looks reasonable to me. I do worry a bit about code duplication.

I was initially concerned about this when splitting try_to_unmap_one() up but  
most of the code paths ended up being pretty orthogonal and I think the 
clarity gained from separating them is worth a small amount of duplication.

> At some point in the future, it might be discovered that other combinations
> of TTU_XXX flags are needed in which case a careful check of 
try_to_migrate()
> and try_to_unmap() will be needed.

I wanted to keep the code as simple as possible by removing all dead code 
paths that that weren't in use today.

I think this is likely to be more of an issue if new TTU_XXX flags are added 
as there are already explicit (and hopefully exhaustive) checks in 
try_to_migrate() and try_to_unmap() for unsupported combinations of existing 
flags. To avoid ending up in the same situation again I would rather not have 
more TTU_XXX flags added if at all possible though.

> Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>

Thanks.

 - Alistair