What is the use of flag PF_MEMALLOC

Question

When I am browsing some code in one device driver in linux, I found the flag PF_MEMALLOC is being set in the thread (process). I found the definition of this flag in header file, which saying that "Allocating Memory"

#define PF_MEMALLOC     0x00000800      /* Allocating memory */

So, my doubt here is, what exactly the use of this flag when set it in a process/thread like code current->flags |= PF_MEMALLOC;

Answer 1

This flag is used within the kernel to indicate a thread that is current executing with a memory-allocation path, and therefore is allowed to recursively allocate any memory it requires ignoring watermarks and without being forced to write out dirty pages.

This is to ensure that if the code that is attempting to free pages in order to satisfy an original allocation request itself has to allocate a small amount of memory to proceed, that code won't then recursively try to free pages.

Most drivers should not require this flag.