Were they called serfs in France? I thought serfs were an eastern Europe thing.

There is an article in the "Past & Present" Oxford Journal that details the "Knight's Fee" (http://past.oxfordjournals.org/content/49/1/3.extract) which is unfortunately behind a paywall, but basically a free man who could fight and equip himself with arms and armor could "buy" a knighthood.

See also 1324-Westminster, 10 May. Rot. Claus. 7 Ed.II. m II. d.

York. --Writ, addressed to the sheriff, commanding him to make a proclamation in full County Court, that all persons holding lands to the amount of £40 per annum, or an entire knight's fee of the value of £40 per annum, and who have held the same for three entire years, of whomsoever such lands should be holden, and who are not now knights, shall take upon themselves the degree of knighthood on or before Michaelmas then next. --The Sheriff is to return the names of the individual so qualified, and he is warned that strict enquiry will be made as to his conduct in the execution of the writ.

It should be noted that serfdom was generally considered a condition of bondage, so one would have to rise from serfdom to becoming a freeman, become very successful financially at a young enough age to continue being able to take up arms, and only then could they purchase their way to a knighthood.