Why __alloc_contig_migrate_range calls migrate_prep() at first?

["Wang Sheng-Hui" <shhuiw@xxxxxxxxxxx>]

Dear,

Sorry to trouble you.

I noticed cma_alloc would turn to  __alloc_contig_migrate_range for allocating pages.
But  __alloc_contig_migrate_range calls  migrate_prep() at first, even if the requested page
is single and free, lru_add_drain_all still run (called by  migrate_prep())?

Image a large chunk of free contig pages for CMA, various drivers may request a single page from
the CMA area, we'll get  lru_add_drain_all run for each page.

Should we detect if the required pages are free before migrate_prep(), or detect at least for single 
page allocation?

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Regards,
Wang Sheng-Huiÿôèº{.nÇ+?·ÿ±éÝjg¬±¨Â+aþéì¹»®&Þ?)îÇù¦þ)í?æèw+h?¨è­Ú&£ù/iÉÿ¢¸ÿ?梷¢?ú(?¸§»þÇ?þm§ÿÿÃÿ?)îÇù¦þ?àÿðè?û^?¨¥ÿö¡­ç?¨¥¶?ÿv?íþKÚrOè®ÿzf¢?ù/iÉÿ¢¸ÿ