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V" , Zi Yan , shuah Khan , Dev Jain References: <20241219124717.4907-1-donettom@linux.ibm.com> Content-Language: en-US From: Donet Tom In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: 03vuiCjA-V0Qgf5Yk4pq8FtvDd6iHdPe X-Proofpoint-GUID: ToskKSOkd6mHJudQZOBHIWO1YrXlmpnc X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1051,Hydra:6.0.680,FMLib:17.12.62.30 definitions=2024-10-15_01,2024-10-11_01,2024-09-30_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 malwarescore=0 bulkscore=0 mlxscore=0 priorityscore=1501 suspectscore=0 lowpriorityscore=0 adultscore=0 impostorscore=0 mlxlogscore=792 phishscore=0 spamscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2411120000 definitions=main-2412200020 On 12/19/24 18:28, David Hildenbrand wrote: > On 19.12.24 13:47, Donet Tom wrote: >> The migration selftest is currently failing for shared anonymous >> mappings due to a race condition. >> >> During migration, the source folio's PTE is unmapped by nuking the >> PTE, flushing the TLB,and then marking the page for migration >> (by creating the swap entries). The issue arises when, immediately >> after the PTE is nuked and the TLB is flushed, but before the page >> is marked for migration, another thread accesses the page. This >> triggers a page fault, and the page fault handler invokes >> do_pte_missing() instead of do_swap_page(), as the page is not yet >> marked for migration. >> >> In the fault handling path, do_pte_missing() calls __do_fault() >> ->shmem_fault() -> shmem_get_folio_gfp() -> filemap_get_entry(). >> This eventually calls folio_try_get(), incrementing the reference >> count of the folio undergoing migration. The thread then blocks >> on folio_lock(), as the migration path holds the lock. This >> results in the migration failing in __migrate_folio(), which expects >> the folio's reference count to be 2. However, the reference count is >> incremented by the fault handler, leading to the failure. >> >> The issue arises because, after nuking the PTE and before marking the >> page for migration, the page is accessed. To address this, we have >> updated the logic to first nuke the PTE, then mark the page for >> migration, and only then flush the TLB. With this patch, If the page is >> accessed immediately after nuking the PTE, the TLB entry is still >> valid, so no fault occurs. > > But what about if the PTE is not in the TLB yet, and you get an access > from another CPU just after clearing the PTE (but not flushing the > TLB)? The other CPU will still observe PTE=none, trigger a fault etc. > Yes, in this scenario, the migration will fail. Do you think the migration test failure, even after a retry, should be considered a major issue that must be fixed? > So I don't think what you propose rules out all cases. >