On Thu, 19 Mar 2015, Kirill A. Shutemov wrote: > Currently we take naive approach to page flags on compound -- we set the > flag on the page without consideration if the flag makes sense for tail > page or for compound page in general. This patchset try to sort this out > by defining per-flag policy on what need to be done if page-flag helper > operate on compound page. > > The last patch in patchset also sanitize usege of page->mapping for tail > pages. We don't define meaning of page->mapping for tail pages. Currently > it's always NULL, which can be inconsistent with head page and potentially > lead to problems. > > For now I catched one case of illigal usage of page flags or ->mapping: > sound subsystem allocates pages with __GFP_COMP and maps them with PTEs. > It leads to setting dirty bit on tail pages and access to tail_page's > ->mapping. I don't see any bad behaviour caused by this, but worth fixing > anyway. But there's nothing to fix there. We're more used to having page->mapping set by filesystems, but it is normal for drivers to have pages with NULL page->mapping mapped into userspace (and it's not accidental that they appear !PageAnon); and subpages of compound pages mapped into userspace, and set_page_dirty applied to them. > > This patchset makes more sense if you take my THP refcounting into > account: we will see more compound pages mapped with PTEs and we need to > define behaviour of flags on compound pages to avoid bugs. Yes, I quite understand that you want to clarify the usage of different page flags to yourself, to help towards a policy of what to do with each of them when subpages of a huge compound page are mapped into userspace; but I don't see that we need this patchset in the kernel now, given that it adds unnecessary overhead into several low-level inline functions. I'm surprised that Andrew has fast-tracked it into his mmotm tree: I don't think it's harmful beyond the overhead, but it seems premature: let's wait until we get some benefit too? Hugh -- 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>