On Fri 02-10-15 15:35:49, Vlastimil Babka wrote: > Currently, /proc/pid/smaps will always show "Swap: 0 kB" for shmem-backed > mappings, even if the mapped portion does contain pages that were swapped out. > This is because unlike private anonymous mappings, shmem does not change pte > to swap entry, but pte_none when swapping the page out. In the smaps page > walk, such page thus looks like it was never faulted in. > > This patch changes smaps_pte_entry() to determine the swap status for such > pte_none entries for shmem mappings, similarly to how mincore_page() does it. > Swapped out pages are thus accounted for. > > The accounting is arguably still not as precise as for private anonymous > mappings, since now we will count also pages that the process in question never > accessed, but only another process populated them and then let them become > swapped out. I believe it is still less confusing and subtle than not showing > any swap usage by shmem mappings at all. Also, swapped out pages only becomee a > performance issue for future accesses, and we cannot predict those for neither > kind of mapping. > > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> > Acked-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> But I think comments explaining why i_mutex is not needed are confusing and incomplete. [...] > + /* > + * Here we have to inspect individual pages in our mapped range to > + * determine how much of them are swapped out. Thanks to RCU, we don't > + * need i_mutex to protect against truncating or hole punching. > + */ > + start = linear_page_index(vma, vma->vm_start); > + end = linear_page_index(vma, vma->vm_end); > + > + return shmem_partial_swap_usage(inode->i_mapping, start, end); [...] > +/* > + * Determine (in bytes) how many pages within the given range are swapped out. > + * > + * Can be called without i_mutex or mapping->tree_lock thanks to RCU. > + */ > +unsigned long shmem_partial_swap_usage(struct address_space *mapping, > + pgoff_t start, pgoff_t end) AFAIU RCU only helps to prevent from accessing nodes which were freed from the radix tree. The reason why we do not need to hold i_mutex is that the radix tree iterator would break out of the loop if we entered node which backed truncated range. At least this is my understanding, I might be wrong here of course. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>