| Message ID | 20160211181306.7864.44244.stgit@maxim-thinkpad (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 02/12/2016 05:48 AM, Dmitry Monakhov wrote: > Maxim Patlasov <mpatlasov@virtuozzo.com> writes: > >> The patch solves the following problem: file system specific routines >> involved in ordinary routine writeback process BUG_ON page_buffers() >> because a page goes to writeback without buffer-heads attached. >> >> The way how kvm_set_pfn_dirty calls SetPageDirty works only for anon >> mappings. For file mappings it is obviously incorrect - there page_mkwrite >> must be called. It's not easy to add page_mkwrite call to kvm_set_pfn_dirty >> because there is no universal way to find vma by pfn. But actually >> SetPageDirty may be simply skipped in those cases. Below is a >> justification. > Confirm. I've hit that BUGON > [ 4442.219121] ------------[ cut here ]------------ > [ 4442.219188] kernel BUG at fs/ext4/inode.c:2285! > <...> > >> When guest modifies the content of a page with file mapping, kernel kvm >> makes the page dirty by the following call-path: >> >> vmx_handle_exit -> >> handle_ept_violation -> >> __get_user_pages -> >> page_mkwrite -> >> SetPageDirty >> >> Since then, the page is dirty from both guest and host point of view. Then >> the host makes writeback and marks the page as write-protected. So any >> further write from the guest triggers call-path above again. > Please elaborate exact call-path which marks host-page. wb_workfn -> wb_do_writeback -> wb_writeback -> __writeback_inodes_wb -> writeback_sb_inodes -> __writeback_single_inode -> do_writepages -> ext4_writepages -> mpage_prepare_extent_to_map -> mpage_process_page_bufs -> mpage_submit_page -> clear_page_dirty_for_io -> page_mkclean -> rmap_walk-> rmap_walk_file -> page_mkclean_one-> pte_wrprotect -> pte_clear_flags(pte, _PAGE_RW) Thanks, Maxim >> So, for file mappings, it's not possible to have new data written to a page >> inside the guest w/o corresponding SetPageDirty on the host. >> >> This makes explicit SetPageDirty from kvm_set_pfn_dirty redundant. >> >> Signed-off-by: Maxim Patlasov <mpatlasov@virtuozzo.com> >> --- >> virt/kvm/kvm_main.c | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c >> index a11cfd2..5a7d3fa 100644 >> --- a/virt/kvm/kvm_main.c >> +++ b/virt/kvm/kvm_main.c >> @@ -1582,7 +1582,8 @@ void kvm_set_pfn_dirty(kvm_pfn_t pfn) >> if (!kvm_is_reserved_pfn(pfn)) { >> struct page *page = pfn_to_page(pfn); >> >> - if (!PageReserved(page)) >> + if (!PageReserved(page) && >> + (!page->mapping || PageAnon(page))) >> SetPageDirty(page); >> } >> } >> >> _______________________________________________ >> Linux-nvdimm mailing list >> Linux-nvdimm@lists.01.org >> https://lists.01.org/mailman/listinfo/linux-nvdimm
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index a11cfd2..5a7d3fa 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1582,7 +1582,8 @@ void kvm_set_pfn_dirty(kvm_pfn_t pfn) if (!kvm_is_reserved_pfn(pfn)) { struct page *page = pfn_to_page(pfn); - if (!PageReserved(page)) + if (!PageReserved(page) && + (!page->mapping || PageAnon(page))) SetPageDirty(page); } }
The patch solves the following problem: file system specific routines involved in ordinary routine writeback process BUG_ON page_buffers() because a page goes to writeback without buffer-heads attached. The way how kvm_set_pfn_dirty calls SetPageDirty works only for anon mappings. For file mappings it is obviously incorrect - there page_mkwrite must be called. It's not easy to add page_mkwrite call to kvm_set_pfn_dirty because there is no universal way to find vma by pfn. But actually SetPageDirty may be simply skipped in those cases. Below is a justification. When guest modifies the content of a page with file mapping, kernel kvm makes the page dirty by the following call-path: vmx_handle_exit -> handle_ept_violation -> __get_user_pages -> page_mkwrite -> SetPageDirty Since then, the page is dirty from both guest and host point of view. Then the host makes writeback and marks the page as write-protected. So any further write from the guest triggers call-path above again. So, for file mappings, it's not possible to have new data written to a page inside the guest w/o corresponding SetPageDirty on the host. This makes explicit SetPageDirty from kvm_set_pfn_dirty redundant. Signed-off-by: Maxim Patlasov <mpatlasov@virtuozzo.com> --- virt/kvm/kvm_main.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)