Date: Fri, 6 Mar 2015 15:19:17 +0000 (UTC) From: Gleb Smirnoff <glebius@FreeBSD.org> To: src-committers@freebsd.org, svn-src-projects@freebsd.org Subject: svn commit: r279690 - projects/sendfile/sys/kern Message-ID: <201503061519.t26FJHQW096128@svn.freebsd.org>
next in thread | raw e-mail | index | archive | help
Author: glebius Date: Fri Mar 6 15:19:16 2015 New Revision: 279690 URL: https://svnweb.freebsd.org/changeset/base/279690 Log: Provide extra comments to explain better how new sendfile works. Sponsored by: Nginx, Inc. Sponsored by: Netflix Modified: projects/sendfile/sys/kern/uipc_syscalls.c Modified: projects/sendfile/sys/kern/uipc_syscalls.c ============================================================================== --- projects/sendfile/sys/kern/uipc_syscalls.c Fri Mar 6 14:24:37 2015 (r279689) +++ projects/sendfile/sys/kern/uipc_syscalls.c Fri Mar 6 15:19:16 2015 (r279690) @@ -2055,6 +2055,19 @@ fixspace(int old, int new, off_t off, in KASSERT(*space >= 0, ("%s: space went backwards", __func__)); } +/* + * Structure describing a single sendfile(2) I/O, which may consist of + * several underlying pager I/Os. + * + * The syscall context allocates the structure and initializes 'nios' + * to 1. As sendfile_swapin() runs through pages and starts asynchronous + * paging operations, it increments 'nios'. + * + * Every I/O completion calls sf_iodone(), which decrements the 'nios', and + * the syscall also calls sf_iodone() after allocating all mbufs, linking them + * and sending to socket. Whoever reaches zero 'nios' is responsible to + * call pru_ready on the socket, to notify it of readyness of the data. + */ struct sf_io { u_int nios; u_int error; @@ -2110,6 +2123,9 @@ sf_iodone(void *arg, vm_page_t *pg, int free(sfio, M_TEMP); } +/* + * Iterate through pages vector and request paging for non-valid pages. + */ static int sendfile_swapin(vm_object_t obj, struct sf_io *sfio, off_t off, off_t len, int npages, int rhpages) @@ -2119,6 +2135,11 @@ sendfile_swapin(vm_object_t obj, struct nios = 0; VM_OBJECT_WLOCK(obj); + + /* + * First grab all the pages and wire them. Note that we grab + * only required pages. Readahead pages are dealt with later. + */ for (int i = 0; i < npages; i++) pa[i] = vm_page_grab(obj, OFF_TO_IDX(vmoff(i, off)), VM_ALLOC_WIRED | VM_ALLOC_NORMAL); @@ -2126,6 +2147,7 @@ sendfile_swapin(vm_object_t obj, struct for (int i = 0; i < npages;) { int j, a, count, rv; + /* Skip valid pages. */ if (vm_page_is_valid(pa[i], vmoff(i, off) & PAGE_MASK, xfsize(i, npages, off, len))) { vm_page_xunbusy(pa[i]); @@ -2133,11 +2155,26 @@ sendfile_swapin(vm_object_t obj, struct continue; } + /* + * Now 'i' points to first invalid page, iterate further + * to make 'j' point at first valid after a bunch of + * invalid ones. + */ for (j = i + 1; j < npages; j++) if (vm_page_is_valid(pa[j], vmoff(j, off) & PAGE_MASK, xfsize(j, npages, off, len))) break; + /* + * Now we got region of invalid pages between 'i' and 'j'. + * Check that they belong to pager. They may not be there, + * which is a regular situation for shmem pager. For vnode + * pager this happens only in case of sparse file. + * + * Important feature of vm_pager_has_page() is the hint + * stored in 'a', about how many pages we can pagein after + * this page in a single I/O. + */ while (!vm_pager_has_page(obj, OFF_TO_IDX(vmoff(i, off)), NULL, &a) && i < j) { pmap_zero_page(pa[i]); @@ -2149,6 +2186,15 @@ sendfile_swapin(vm_object_t obj, struct if (i == j) continue; + /* + * We want to pagein as many pages as possible, limited only + * by the 'a' hint and actual request. + * + * If calculated count yields in value greater than npages, + * then we are doing optional readahead, and we need to grab + * and wire pages for it. Since readahead is optional, we + * prefer failure over sleep and thus say VM_ALLOC_NOWAIT. + */ count = min(a + 1, npages + rhpages - i); for (j = npages; j < i + count; j++) { pa[j] = vm_page_grab(obj, OFF_TO_IDX(vmoff(j, off)), @@ -2518,20 +2564,18 @@ retry_space: break; } + m0 = m_get(M_WAITOK, MT_DATA); + m0->m_ext.ext_buf = (char *)sf_buf_kva(sf); + m0->m_ext.ext_size = PAGE_SIZE; + m0->m_ext.ext_arg1 = sf; + m0->m_ext.ext_arg2 = sfs; /* - * Get an mbuf and set it up. - * * SF_NOCACHE sets the page as being freed upon send. * However, we ignore it for the last page in 'space', * if the page is truncated, and we got more data to * send (rem > space), or if we have readahead * configured (rhpages > 0). */ - m0 = m_get(M_WAITOK, MT_DATA); - m0->m_ext.ext_buf = (char *)sf_buf_kva(sf); - m0->m_ext.ext_size = PAGE_SIZE; - m0->m_ext.ext_arg1 = sf; - m0->m_ext.ext_arg2 = sfs; if ((flags & SF_NOCACHE) == 0 || (i == npages - 1 && ((off + space) & PAGE_MASK) && @@ -2592,6 +2636,12 @@ retry_space: CURVNET_SET(so->so_vnet); if (nios == 0) { + /* + * If sendfile_swapin() didn't initiate any I/Os, + * which happens if all data is cached in VM, then + * we can send data right now without the + * PRUS_NOTREADY flag. + */ free(sfio, M_TEMP); error = (*so->so_proto->pr_usrreqs->pru_send) (so, 0, m, NULL, NULL, td);
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?201503061519.t26FJHQW096128>