Re: [patch, feature] nonlinear mappings, prefaulting support, 2.5.42-F8

Ingo Molnar
2002-10-14T15:02:26Z

On Mon, 14 Oct 2002, William Lee Irwin III wrote:

> > +int sys_remap_file_pages(unsigned long start, unsigned long size,
> > + unsigned long prot, unsigned long pgoff, unsigned long flags)
>
> ISTR suggesting vectorizing this to reduce syscall traffic.

well, i dont agree with vectorizing everything, unless it's some really
lightweight thing and/or the operation is somehow naturally vectorized.
(block and network IO, etc.)

> The semantics of faulting in pages on such a region, while not
> incorrect, are at least unusual enough to raise the question of whether
> it's appropriate for the kernel to fill the pages as opposed to
> returning an error to userspace. [...]

the pagefault path simply does not have the information whether a mapping
was nonlinear, possibly long before. In the initial patch i had a
VM_RANDOM flag for vmas, which was set up at mmap() time, but this
restricted the API needlessly. Tracking whether a mapping was remapped
randomly before does not sound too useful to me either. So right now it's
the responsibility of the user to use the API in a meaningful way - is it
such a big problem?

> [...] The requirement of MAP_LOCKED or PROT_NONE might as well be
> in-kernel if the file offset contiguity assumption is not met, [...]

how would you do this actually, without other restrictions?

> sys_remap_file_pages() also interacts in an unusual way with the
> semantics of MAP_POPULATE. MAP_POPULATE seems to perform a non-blocking
> make_pages_present() operation not shared with MAP_LOCKED, [...]

what it does is a blocking make_pages_present(), for nonblocking you also
need to specify MAP_NONBLOCK. I agree that the mlock path should/could be
merged with the populate path, this was suggested by Linus as well.

> [...] and filemap_populate() performs the file offset contiguous
> prefaulting which again doesn't mix well with the scatter gather
> semantics desired.

huh?

> Also, a stranger phenomenon appears in filemap_populate(), where
> nonblock may be true, and so filemap_getpage() will return NULL, but
> -ENOMEM is returned if filemap_getpage() returns NULL.

true, this is a bug, i fixed it in the -F9 patch at:

http://redhat.com/~mingo/remap-file-pages-patches/

> Also, I see a significant portion of filemap_nopage() duplicated in
> filemap_getpage(), including long-stale hashtable-related comments.

check the announcement email for details about the seemingly duplicated
code of filemap_nopage() and filemap_getpage(). And which hashing comments
do you mean? We still hash pagecache pages.

Ingo