Re: page migration: Fail with error if swap not setup

[Andrew Morton <akpm@osdl.org>]

Christoph Lameter <clameter@sgi.com> wrote:
>
> On Tue, 14 Mar 2006, Andrew Morton wrote:
> 
> > Christoph Lameter <clameter@sgi.com> wrote:
> > >
> > > Currently the migration of anonymous pages will silently fail if no swap 
> > > is setup.
> > 
> > Why?
> 
> The allocation of the swap page will fail in migrate_pages() and then the 
> page is going on the permant failure list. Hmm... This is not a real 
> total failure of page migration since file backed pages can be migrated 
> without having swap and page migration will continue for those. However, 
> all anonymous pages will end up on the failed list. At the end of page 
> migration these will be returned to the LRU. Thus they stay where they 
> were.
> 
> > I mean, if something tries to allocate a swap page and that fails then the
> > error should be propagated back.  That's race-free.
> 
> It is propaged back in the form of a list of pages that failed to migrate. 
> Its just no clear at the end what the reasons for the individual failures
> were. Its better just to check for swap availability before migration.

But the operation can still fail if we run out of swapspace partway through
- so this problem can still occur.  The patch just makes it (much) less
frequent.

Surely it's possible to communicate -ENOSWAP correctly and reliably?

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