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IOW, unlike split done in place, > shatter is the exact opposite of collapse. > > The advantage of shattering is that it keeps the original THP intact. Why keep the THP intact? To prevent the THP from fragmentation, since the shattered part will not be returned to buddy allocator for reuse? I agree with the idea of shattering, but keeping THP intact might give us trouble for 1GB THP case when PMD mapping is created after shattering. How to update mapcount for a PMD mapping in the middle of a 1GB folio? I used head[0], head[512], ... as the PMD mapping head page, but that is ugly. For mTHPs, there is no such problem since only PTE mappings are involved. It might be better to just split the THP and move free pages to a donot-use free list until the rest are freed too, if the zone enforces a minimal order that is larger than the free pages. > The cost of copying during the migration is not a side effect, but > rather by design, since splitting is considered a discouraged > behavior. In retail terms, the return of a purchase is charged with a > restocking fee and the original goods can be resold. > > THPs from ZONE_NOMERGE can only be shattered, since they cannot be > split or merged. THPs from ZONE_NOSPLIT can be shattered or split (the > latter requires [1]), if they are above the minimum order. > > [1] https://lore.kernel.org/20240226205534.1603748-1-zi.yan@sent.com/ > -- Best Regards, Yan, Zi --=_MailMate_BF2BC392-AB01-4E9A-BA9D-892E79F7FEB7_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEE6rR4j8RuQ2XmaZol4n+egRQHKFQFAmXg/UwPHHppeUBudmlk aWEuY29tAAoJEOJ/noEUByhU0wIQAJWs9RW8WNKqy3Hwu8vE0aekm8niQ+H5ce5q 3aloonl7D8LxOpxiWTebNhXvJCt5zYpBOnyxVLl/6HJtZHLJ/JYR4rtoQ0QY7dyN BVJd519IKdHB8wbVBRbrjwm1hrOiHU+WdW0y5q+WGHg4LSjnEhNRLPQ9McgNMYIO pGyclTzq4MQtEsyLRN7octgL3VwW3M6IQNrfDtlpO9MxpJnDSf55dCPwK3JC9WXf +7OPCeSJoj7ecs+1WhPZGGR49cUUsKRP7bo/jmIwVRegm5NBVajXa9/4S4yC1GUW 2YsJXvqik+Qsv6b45gbc20cPaYcJLpOjAs3+D2hNSlwyBmZhfWf4aRppXJDDdBH6 HeN5OMc4+6/ltR/i7NoZEtk/QrDlN8rGU7q6EyCKsRszwsddL7+reex2AKVMxXTP k2A3JawLT2yZP+hnDum3J/aNk8Bo6joezFGoMBxXiV+sABmYY3ha3yp309qcZ+Tj ORAt564CbYCTYet8IeFpF1/o1jnvuu+r7uAM+ZXkndHCRga3UF/aoM+F/I6gH1SU q8BRICYg2O8Ivai9ulEvh6sUgPRKSV2AFXFYen7QQ28Z7d3fV0v/WLq5Q0vqE7TO m1oIyAtkAH3c7UKAp+gEz9O0h3DIlZhZkPprTn4Px0cXBGya4JruoeTdhopWtZ6D 7a9ZQgy1 =mywY -----END PGP SIGNATURE----- --=_MailMate_BF2BC392-AB01-4E9A-BA9D-892E79F7FEB7_=--