On Sun, 27 Jan 2013, Simon Jeons wrote: > On Sun, 2013-01-27 at 15:12 -0800, Hugh Dickins wrote: > > On Sat, 26 Jan 2013, Simon Jeons wrote: > > > > > > Could you explain why need check page->mapping twice after get page? > > > > Once for the !locked case, which should not return page if mapping changed. > > Once for the locked case, which should not return page if mapping changed. > > We could use "else", but that wouldn't be an improvement. > > But for locked case, page->mapping will be check twice. Thrice. I'm beginning to wonder: you do realize that page->mapping is volatile, from the point of view of get_ksm_page()? That is the whole point of why get_ksm_page() exists. I can see that the word "volatile" is not obviously used here - it's tucked away inside the ACCESS_ONCE() - but I thought the descriptions of races and barriers made that obvious. If the comments here haven't helped enough, please take a look at git commit 4035c07a8959 "ksm: take keyhole reference to page". -- 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>