svn commit: r288431 - in head/sys: kern sys vm

[John-Mark Gurney]

Alan Cox wrote this message on Fri, Oct 02, 2015 at 18:50 -0500:
> On Oct 2, 2015, at 10:59 AM, John Baldwin <jhb at freebsd.org> wrote:
> 
> > I think it is not unreasonble to expect that fadvise() incurs system-wide
> > affects.  A properly implemented WILLNEED that does read-ahead cannot work
> > without incurring system-wide effects.  I had always assumed that fadvise()
> > operated on a file, not a given process' view of a file (unlike, say,
> > madvise which only operates on mappings and only indirectly affects
> > file-backed data).
> 
> 
> Can you elaborate on what you mean by ?I had always assumed that fadvise() operated on a file, ???
> 
> Under the previous implementation, if you did an fadvise(DONTNEED) on a file, in order to cache the file?s pages, those pages first had to be unmapped from any address space.  (You can find this unmapping performed by vm_page_try_to_cache().)  In other words, there was never any code that said, ?Is this a mapped page, and if it is, don?t cache it because we?re actually performing an fadvise().?  So, to pick an extreme example, if you did an fadvise(?libc.so?, DONTNEED), unless some process had libc.so wired, then every single mapping to every single page of libc.so was going to be destroyed and the pages moved to the cache.  However, because we moved the pages to the cache (rather than freeing them), and libc.so is frequently accessed, a subsequent instruction fetch would have faulted and been able to reactivate the cached page, avoiding an I/O operation.  In other words, that we were caching the pages targeted by fadvise() rather than simply freeing them mattered in cases where the pages were in use/accessed by multiple processes.

This would be a very nasty DoS if someone just ran fadvise('libc.so',
DONTNEED) in loop, and forced any future accesses of libc.so to pull
from disk, over and over and over again...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."