Messages in this thread | | | Date | Tue, 24 Apr 2007 19:53:54 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: Pagecache: find_or_create_page does not call a proper page allocator function |
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On Tue, 24 Apr 2007, Christoph Lameter wrote: > On Tue, 24 Apr 2007, Hugh Dickins wrote: > > > I've not yet looked at the patch under discussion, but this remark > > prompts me... a couple of days ago I got very worried by the various > > hard-wired GFP_HIGHUSER allocations in mm/migrate.c and mm/mempolicy.c, > > and wondered how those would work out if someone has a blockdev mmap'ed. > > I hope you are not confused by the fact that memory policies are only > ever applied to one zone on a node. This is either HIGHMEM or NORMAL. > There is no memory policy support for other than the highest zone.
I was certainly ignorant of that; but I'm not convinced it eliminates the potential issue. For a start, sys_move_pages seems not to involve mempolicies at all - I don't see what prevents it migrating blockdev pages away from the only node which has NORMAL memory.
> Metadata is not movable nor subject to memory policies. > It will never be mapped into a process space.
Not as metadata, no. But someone (let's hope only root, though I may be wrong on that) can map any part of the block device into userspace.
> > yup. wherever we dereference buffer_head.b_data we're touching > > page_address(buffer_head.b_page) without kmapping. > > Yes but before we get there we will bounce pagecache pages into an area > where we do not need kmap.
Again, I'm not convinced: bouncing gets done for the I/O, but where is it done to meet the filesystem's expectations?
On the other hand, as I said, I've seen no problem myself in practice. However, if there is no problem, why do block devices demand GFP_USER?
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