On Fri, Feb 10, 2017 at 02:35:05PM +0100, Michal Hocko wrote: > On Fri 03-02-17 15:33:23, Shaohua Li wrote: > > Add a separate RSS for MADV_FREE pages. The pages are charged into > > MM_ANONPAGES (because they are mapped anon pages) and also charged into > > the MM_LAZYFREEPAGES. /proc/pid/statm will have an extra field to > > display the RSS, which userspace can use to determine the RSS excluding > > MADV_FREE pages. > > > > The basic idea is to increment the RSS in madvise and decrement in unmap > > or page reclaim. There is one limitation. If a page is shared by two > > processes, since madvise only has mm cotext of current process, it isn't > > convenient to charge the RSS for both processes. So we don't charge the > > RSS if the mapcount isn't 1. On the other hand, fork can make a > > MADV_FREE page shared by two processes. To make things consistent, we > > uncharge the RSS from the source mm in fork. > > > > A new flag is added to indicate if a page is accounted into the RSS. We > > can't use SwapBacked flag to do the determination because we can't > > guarantee the page has SwapBacked flag cleared in madvise. We are > > reusing mappedtodisk flag which should not be set for Anon pages. > > > > There are a couple of other places we need to uncharge the RSS, > > activate_page and mark_page_accessed. activate_page is used by swap, > > where MADV_FREE pages are already not in lazyfree state before going > > into swap. mark_page_accessed is mainly used for file pages, but there > > are several places it's used by anonymous pages. I fixed gup, but not > > some gpu drivers and kvm. If the drivers use MADV_FREE, we might have > > inprecise RSS accounting. > > > > Please note, the accounting is never going to be precise. MADV_FREE page > > could be written by userspace without notification to the kernel. The > > page can't be reclaimed like other clean lazyfree pages. The page isn't > > real lazyfree page. But since kernel isn't aware of this, the page is > > still accounted as lazyfree, thus the accounting could be incorrect. > > This is all quite complex and as you say unprecise already. From the > description it is not even clear why do we need it at all. Why is > /proc/<pid>/smaps insufficient? I am also not fun of a new page flag - > even though you managed to recycle an existing one which is a plus. We have monitor app running in the system to check other apps' RSS and kill them if RSS is abnormal. Checking /proc/pid/smaps is too complicated and slow, don't think we can go that way. Yes, the accounting isn't precise, but should be much better than exporting nothing to userspace. Thanks, Shaohua -- 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>
