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? Hugh - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/