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V" , Zi Yan , shuah Khan , Dev Jain References: <20241219124717.4907-1-donettom@linux.ibm.com> <9bc000ab-1982-41f0-9ca0-2a4ead9aa982@linux.ibm.com> <454f7c85-a6d9-4635-94ff-7fab69060a82@redhat.com> Content-Language: en-US From: Donet Tom In-Reply-To: <454f7c85-a6d9-4635-94ff-7fab69060a82@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: 6QLzgylWL-rjejHBhERTDns7hUrlHEFr X-Proofpoint-ORIG-GUID: DBpBtrrnYMDtTgdVM3vgO-93NvmfgG3b 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 mlxlogscore=659 phishscore=0 impostorscore=0 malwarescore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 suspectscore=0 bulkscore=0 adultscore=0 priorityscore=1501 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2411120000 definitions=main-2412230108 On 12/20/24 15:41, David Hildenbrand wrote: > On 20.12.24 03:55, Donet Tom wrote: >> >> 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? > > I think it is something we should definitely improve, but I think our > page migration should handle this in a better way, not the unmap > logic. I recall we discussed with Dev some ideas on how to improve that? > > > I'm pretty sure one can trigger similar case using a tmpfs file and > using read/write in a loop instead of memory access -> page faults. So > where racing with page faults is completely out of the picture. Thank you David. I will try this scenario as well and come back with some ideas for improvement.