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Fri, 31 Jul 2026 19:37:50 -0700 (PDT) Received: from setun ([223.228.61.127]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-3153dd4e47dsm15678889eec.3.2026.07.31.19.37.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 31 Jul 2026 19:37:49 -0700 (PDT) From: Aadeshveer Singh To: qemu-devel@nongnu.org Cc: peterx@redhat.com, farosas@suse.de, pbonzini@redhat.com, philmd@mailo.com, lvivier@redhat.com, ayoub@saferwall.com, pierrick.bouvier@oss.qualcomm.com, Aadeshveer Singh Subject: [PATCH v4 07/11] migration: add support for fault thread to load pages from disk Date: Sat, 1 Aug 2026 08:06:24 +0530 Message-ID: <20260801023628.22665-8-aadeshveer07@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260801023628.22665-1-aadeshveer07@gmail.com> References: <20260801023628.22665-1-aadeshveer07@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2607:f8b0:4864:20::631; envelope-from=aadeshveer07@gmail.com; helo=mail-pl1-x631.google.com X-Spam_score_int: -12 X-Spam_score: -1.3 X-Spam_bar: - X-Spam_report: (-1.3 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_ENVFROM_END_DIGIT=0.25, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URG_BIZ=0.573 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org In fast snapshot load, we would like to serve faults as soon as possible hence loading pages directly instead of requesting a source Add postcopy_mapped_ram_load_page() function which serves single page fault. It uses bitmap_test_and_clear_atomic() on pending_bmap to prevent multiple threads from loading same page. It loads a page or pages depending on size of guest pages and host pages, loading exactly the larger of two. If the entire page is zero postcopy_place_page_zero() is used for efficiency and in case some part is non zero it is part by part loaded on loop using new function postcopy_mapped_ram_load_guest_page() which loads a single guest page in a buffer which is then placed using postcopy_place_page(). This covers all possible cases for various page sizes of host and guest. Update postcopy_ram_fault_thread to call postcopy_mapped_ram_load_page instead of requesting source in case of fast snapshot load. to_src_file check is bypassed in fast snapshot load case as there is no source. Call try_mark_postcopy_blocktime_begin on every page fault to support postcopy-blocktime. Allocate another channel in postcopy_temp_pages_setup(like the preempt case), for both the fault thread and eager thread to load pages independently. Signed-off-by: Aadeshveer Singh --- migration/postcopy-ram.c | 174 +++++++++++++++++++++++++++++++++++---- 1 file changed, 157 insertions(+), 17 deletions(-) diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c index 2e2c9fae10..1da9c4309b 100644 --- a/migration/postcopy-ram.c +++ b/migration/postcopy-ram.c @@ -949,6 +949,121 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd, pagesize); } +/* + * Load a single guest page from source file into the buffer. + * NOTE: This is not an atomic operation and should not be used to directly load + * pages on page faults in postcopy. It is meant to fill in buffer that can then + * be copied into the faulting location using UFFDIO_COPY. + */ +static bool postcopy_mapped_ram_load_guest_page(MigrationIncomingState *mis, + RAMBlock *rb, + ram_addr_t rb_offset, void *buf, + Error **errp) +{ + ERRP_GUARD(); + size_t page = rb_offset / qemu_target_page_size(); + size_t read; + + if (test_bit(page, rb->file_bmap)) { + /* + * This can happen concurrently, but it's thread-safe because + * qemu_get_buffer_at() is thread-safe, and the caller will be using + * different temporary buffers. + */ + read = + qemu_get_buffer_at(mis->from_src_file, buf, qemu_target_page_size(), + rb->pages_offset + rb_offset, errp); + + if (read != qemu_target_page_size()) { + error_prepend(errp, + "Could not read page %zu from RAM Block %s: ", page, + rb->idstr); + return false; + } + } else { + memset(buf, '\0', qemu_target_page_size()); + } + return true; +} + +/** + * postcopy_mapped_ram_load_page() - Load pages required to access host address. + * @mis: Migration Incoming State. + * @rb: RAMBlock from where page is loaded. + * @rb_offset: Offset of target page in RAMBlock. + * @haddr: Base of target page where to load in page. + * @channel: Used to identify between threads and use corresponding temp. + * @errp: Set error in case of failure + * + * Load page(s) from RAMBlock covering the faulting address. We might need to + * load multiple pages in the case when host page size is greater than guest + * page size. As userfaultfd works on granularity of host pages, we might need + * to load guest pages in single operation. + * + * Return: True on success. + */ +static bool postcopy_mapped_ram_load_page(MigrationIncomingState *mis, + RAMBlock *rb, ram_addr_t rb_offset, + uint64_t haddr, int channel, + Error **errp) +{ + void *place_source = mis->postcopy_tmp_pages[channel].tmp_huge_page; + char *buffer_ptr = (char *)place_source; + size_t guest_pages_to_load = + MAX(1, qemu_ram_pagesize(rb) / qemu_target_page_size()); + size_t host_page; + size_t page; + + /* + * If guest page size is greater than host page size uffd needs to load one + * guest page and multiple host pages, hence the offsets need to aligned + * with guest pages (which is automatically aligned with host pages). In the + * same case we need to check range of bits on pending_bmap(bit per host + * page) to decide whether all the page have been loaded + */ + rb_offset = ROUND_DOWN(rb_offset, qemu_target_page_size()); + haddr = ROUND_DOWN(haddr, qemu_target_page_size()); + host_page = rb_offset / qemu_ram_pagesize(rb); + page = rb_offset >> qemu_target_page_bits(); + + if (bitmap_test_and_clear_atomic( + rb->pending_bmap, host_page, + MAX(1, qemu_target_page_size() / qemu_ram_pagesize(rb)))) { + if (find_next_bit(rb->file_bmap, page + guest_pages_to_load, page) == + page + guest_pages_to_load) { + /* It is efficient to use UFFDIO_ZERO if all pages are zero */ + if (postcopy_place_page_zero(mis, (void *)haddr, rb)) { + error_setg(errp, + "Failed to place zero page %zu from RAM Block %s at " + "address %" PRIu64, + page, rb->idstr, haddr); + return false; + } + } else { + size_t load_size = guest_pages_to_load * qemu_target_page_size(); + size_t offset; + + for (offset = 0; offset < load_size; + offset += qemu_target_page_size()) { + if (!postcopy_mapped_ram_load_guest_page( + mis, rb, rb_offset + offset, buffer_ptr + offset, + errp)) { + return false; + } + } + + if (postcopy_place_page(mis, (void *)haddr, place_source, rb)) { + error_setg(errp, + "Failed to place page %zu from RAM Block %s at " + "address %" PRIu64, + page, rb->idstr, haddr); + return false; + } + } + } + return true; +} + /* * NOTE: @tid is only used when postcopy-blocktime feature is enabled, and * also optional: when zero is provided, the fault accounting will be ignored. @@ -1310,6 +1425,7 @@ static void *postcopy_ram_fault_thread(void *opaque) int ret; size_t index; RAMBlock *rb = NULL; + Error *local_err = NULL; trace_postcopy_ram_fault_thread_entry(); rcu_register_thread(); @@ -1351,11 +1467,13 @@ static void *postcopy_ram_fault_thread(void *opaque) break; } - if (!mis->to_src_file) { + if (!migrate_mapped_ram() && !mis->to_src_file) { /* - * Possibly someone tells us that the return path is - * broken already using the event. We should hold until - * the channel is rebuilt. + * Possibly someone tells us that the return path is broken already + * using the event. We should hold until the channel is rebuilt. + * Fast snapshot load doesn't support pause and recover, because + * it's not necessary: we can fail right away when QEMU just booted + * with nothing to lose. */ postcopy_pause_fault_thread(mis); } @@ -1418,18 +1536,37 @@ static void *postcopy_ram_fault_thread(void *opaque) qemu_ram_get_idstr(rb), rb_offset, msg.arg.pagefault.feat.ptid); + + if (migrate_mapped_ram()) { + /* Load page directly in case of fast snapshot load */ + + uintptr_t aligned = (uintptr_t)ROUND_DOWN( + msg.arg.pagefault.address, qemu_ram_pagesize(rb)); + + if (try_mark_postcopy_blocktime_begin( + mis, rb, rb_offset, (uintptr_t)aligned, + msg.arg.pagefault.feat.ptid)) { + if (!postcopy_mapped_ram_load_page( + mis, rb, rb_offset, aligned, RAM_CHANNEL_POSTCOPY, + &local_err)) { + error_report_err(local_err); + break; + } + } + } else { retry: - /* - * Send the request to the source - we want to request one - * of our host page sizes (which is >= TPS) - */ - ret = postcopy_request_page(mis, rb, rb_offset, - msg.arg.pagefault.address, - msg.arg.pagefault.feat.ptid); - if (ret) { - /* May be network failure, try to wait for recovery */ - postcopy_pause_fault_thread(mis); - goto retry; + /* + * Send the request to the source - we want to request one + * of our host page sizes (which is >= TPS) + */ + ret = postcopy_request_page(mis, rb, rb_offset, + msg.arg.pagefault.address, + msg.arg.pagefault.feat.ptid); + if (ret) { + /* May be network failure, try to wait for recovery */ + postcopy_pause_fault_thread(mis); + goto retry; + } } } @@ -1501,8 +1638,11 @@ static int postcopy_temp_pages_setup(MigrationIncomingState *mis, Error **errp) unsigned i, channels; void *temp_page; - if (migrate_postcopy_preempt()) { - /* If preemption enabled, need extra channel for urgent requests */ + if (migrate_postcopy_preempt() || migrate_mapped_ram()) { + /* + * If preemption enabled or it is fast snapshot load, need extra channel + * for urgent requests/faults + */ mis->postcopy_channels = RAM_CHANNEL_MAX; } else { /* Both precopy/postcopy on the same channel */ -- 2.55.0